PERF: faster groupby diff
- [x] closes #16706
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry


As @jreback suggests in the original issue, there is a large perf improvement to groupby.diff if implemented directly as a groupby method. 

The int8/int16 type coercion feels a bit odd, but was needed to retain existing behavior as well as pass existing unit tests that explicitly test those dtypes. Separate issue to discuss that: #45562 

```
       before           after         ratio
     [2db3b0a0]       [6edfb839]
                      <faster-groupby-diff>
-     1.98±0.02ms      1.35±0.02ms     0.68  groupby.GroupByMethods.time_dtype_as_group('object', 'diff', 'transformation', 5)
-         799±9μs          472±6μs     0.59  groupby.GroupByMethods.time_dtype_as_group('object', 'diff', 'transformation', 1)
-         461±5μs        166±0.8μs     0.36  groupby.GroupByMethods.time_dtype_as_group('object', 'diff', 'direct', 1)
-        94.0±4ms       2.22±0.9ms     0.02  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 5)
-        69.1±1ms      1.51±0.02ms     0.02  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 5)
-        69.3±2ms      1.48±0.01ms     0.02  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 5)
-        78.5±4ms      1.45±0.01ms     0.02  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 5)
-        94.0±6ms      1.46±0.01ms     0.02  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 5)
-         122±3ms      1.55±0.01ms     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 5)
-         140±2ms         1.45±0ms     0.01  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 5)
-         140±2ms      1.41±0.02ms     0.01  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 5)
-      66.0±0.7ms          502±5μs     0.01  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 1)
-      64.3±0.5ms         488±10μs     0.01  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 1)
-         106±8ms        767±200μs     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 1)
-        75.9±2ms         497±20μs     0.01  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 1)
-        89.8±3ms        587±200μs     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 5)
-        88.5±1ms          526±5μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 5)
-         102±2ms         552±10μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 1)
-         120±2ms          614±4μs     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 1)
-       161±0.6ms          604±7μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 1)
-         138±3ms          500±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 5)
-         139±3ms          498±5μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 5)
-         165±1ms          502±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 1)
-        66.5±1ms          192±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 1)
-        71.2±1ms          201±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 5)
-         124±3ms          326±2μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 5)
-         119±2ms          306±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 1)
-        78.6±1ms          196±5μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 5)
-      70.1±0.8ms          170±5μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 5)
-        67.4±4ms          159±2μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 1)
-        78.4±1ms          183±2μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 1)
-         108±8ms         249±80μs     0.00  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 1)
-         102±3ms          182±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 1)
-         160±2ms          170±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 1)
-         165±3ms          171±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 1)
```
Might be worth deciding on #45609 before proceeding here. ok let's use this PR. can you rebase. we have sufficient asv's here (for the small dtypes as well)? > ok let's use this PR. can you rebase. we have sufficient asv's here (for the small dtypes as well)?

Added int16 to the GroupByMethods asv. Shows a similar improvement as other dtypes:

```
       before           after         ratio
     [19aa5ab5]       [c4459b4e]
     <main>           <faster-groupby-diff>
-      77.7±0.5ms      1.60±0.02ms     0.02  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 5)
-        88.3±1ms      1.33±0.07ms     0.02  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 5)
-        74.8±1ms          601±3μs     0.01  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 1)
-         109±1ms         577±10μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 1)
-      79.2±0.9ms          308±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 5)
-      75.6±0.9ms          288±6μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 1)
-        89.6±2ms          322±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 5)
-         109±2ms          192±7μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 1)
``` https://github.com/pandas-dev/pandas/runs/5346468863?check_suite_focus=true the asv benches are failing, can you take a look @jreback - green, asv issues fixed thanks @lukemanley very  nice! @lukemanley running asvs comparing main (as of yesterday) to v.1.4.0 i'm seeing some pretty big slowdowns in groupby.diff.  For whatever reason I don't see them here (https://pandas.pydata.org/speed/pandas/index.html#regressions?sort=3&dir=desc).  Any idea what might have happened? hmmm... I still see a significant speedup when comparing main to the latest 1.4.x commit. I just ran the following:

```
asv continuous -f 1.1 upstream/1.4.x upstream/main -b groupby.GroupByMethods
```

```
       before           after         ratio
     [21de060f]       [d86e2006]
                      <main>    
-      75.4±0.8ms      1.52±0.01ms     0.02  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 5)
-        78.6±4ms      1.51±0.02ms     0.02  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 5)
-       89.1±10ms      1.65±0.01ms     0.02  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 5)
-        93.4±1ms      1.35±0.01ms     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 5)
-        97.0±1ms      1.38±0.05ms     0.01  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 5)
-        115±40ms      1.55±0.05ms     0.01  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 5)
-        100±10ms      1.31±0.01ms     0.01  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 5)
-         133±2ms      1.67±0.03ms     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 5)
-         148±2ms      1.37±0.01ms     0.01  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 5)
-         144±1ms      1.27±0.02ms     0.01  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 5)
-        70.7±1ms          550±6μs     0.01  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 1)
-       88.1±20ms         623±10μs     0.01  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 1)
-        76.5±5ms         510±10μs     0.01  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 1)
-         109±2ms         582±80μs     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 1)
-         113±3ms          589±9μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 1)
-         131±1ms         675±20μs     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 1)
-         117±7ms          586±6μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 1)
-        112±40ms          522±5μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 1)
-         172±2ms          639±7μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 1)
-        95.2±1ms         349±10μs     0.00  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 5)
-      94.4±0.8ms          343±1μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 5)
-       90.0±20ms          317±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 5)
-        99.4±7ms          328±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 5)
-         172±2ms          533±9μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 1)
-       94.4±20ms          285±7μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 1)
-      72.9±0.7ms          207±1μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 5)
-      68.9±0.5ms          191±4μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 1)
-         134±1ms          367±7μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 5)
-         133±3ms          329±4μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 1)
-         145±1ms          325±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 5)
-         145±5ms          322±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 5)
-        82.2±3ms          177±5μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 5)
-        76.4±5ms        163±0.9μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 1)
-        102±40ms          202±1μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 5)
-         107±2ms         203±20μs     0.00  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 1)
-       107±0.7ms          188±1μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 1)
-         113±5ms          186±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 1)
-        127±60ms         190±20μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 1)
-         176±9ms          183±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 1)
-         171±2ms          176±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 1)
``` Can you compare against v1.4.0 instead of 1.4.x?  If the slowdown is between 1.4.0 and 1.4.x then it must be for a bugfix, in which case we can ignore it. @jbrockmendel - seeing a similar speedup vs. 1.4.0 as well:

```
       before           after         ratio
     [bb1f6515]       [d86e2006]
     <v1.4.0^0>       <main>    
-        75.3±2ms       2.18±0.7ms     0.03  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 5)
-        80.4±1ms       1.98±0.5ms     0.02  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 5)
-        79.6±2ms       1.73±0.2ms     0.02  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 5)
-        81.5±3ms       1.75±0.2ms     0.02  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 5)
-        94.7±3ms       1.43±0.1ms     0.02  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 5)
-        94.2±2ms      1.31±0.02ms     0.01  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 5)
-         133±2ms       1.79±0.1ms     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 5)
-        95.2±5ms      1.28±0.02ms     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 5)
-         149±8ms      1.41±0.03ms     0.01  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 5)
-      76.7±0.8ms        705±100μs     0.01  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'transformation', 1)
-         148±8ms      1.24±0.01ms     0.01  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 5)
-        74.6±2ms        627±200μs     0.01  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'transformation', 1)
-        73.7±1ms         548±40μs     0.01  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'transformation', 1)
-        73.5±2ms          543±8μs     0.01  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'transformation', 1)
-       111±0.7ms         641±70μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'transformation', 1)
-         129±3ms         698±40μs     0.01  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'transformation', 1)
-         110±4ms         588±20μs     0.01  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'transformation', 1)
-         112±2ms          571±5μs     0.01  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'transformation', 1)
-        76.1±3ms         334±70μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 1)
-        79.3±3ms         343±30μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int16', 'diff', 'direct', 5)
-        92.7±2ms         379±50μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 5)
-         173±2ms         641±20μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'transformation', 1)
-        92.2±3ms          341±5μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 5)
-      92.6±0.7ms         342±20μs     0.00  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 5)
-         135±4ms         405±40μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 5)
-         181±2ms          537±8μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'transformation', 1)
-        75.1±1ms         219±30μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 1)
-        80.5±3ms         226±30μs     0.00  groupby.GroupByMethods.time_dtype_as_field('int', 'diff', 'direct', 5)
-        77.2±5ms          216±3μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 5)
-         130±4ms         346±40μs     0.00  groupby.GroupByMethods.time_dtype_as_field('datetime', 'diff', 'direct', 1)
-        74.6±3ms          192±2μs     0.00  groupby.GroupByMethods.time_dtype_as_field('uint', 'diff', 'direct', 1)
-        80.4±5ms         206±20μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 5)
-        73.3±1ms         185±20μs     0.00  groupby.GroupByMethods.time_dtype_as_field('float', 'diff', 'direct', 1)
-         148±5ms          323±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 5)
-         149±1ms          315±4μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 5)
-         112±6ms         200±20μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int', 'diff', 'direct', 1)
-         107±3ms          190±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('int16', 'diff', 'direct', 1)
-         112±2ms          185±1μs     0.00  groupby.GroupByMethods.time_dtype_as_group('uint', 'diff', 'direct', 1)
-         174±8ms          179±3μs     0.00  groupby.GroupByMethods.time_dtype_as_group('float', 'diff', 'direct', 1)
-         179±9ms          175±2μs     0.00  groupby.GroupByMethods.time_dtype_as_group('datetime', 'diff', 'direct', 1)
``` thanks.  i found a slowdown on my linux machine, will try again on mac and see if i can figure out whats going on