PERF: periodarr_to_dt64arr
- [x] closes #33919
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Re-use `ensure_datetime64ns` for the subset of cases where that is applicable.  I'm at a loss for why we get such a slowdown for the other cases.

```
       before           after         ratio
     [42a6d445]       [a0ece778]
     <master>         <perf-periodarr_to_dt64arr>
+      48.7±0.8ms          133±6ms     2.72  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 4000)
+        48.0±1ms        131±0.8ms     2.72  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 5000)
+        493±10μs      1.33±0.05ms     2.70  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 4000)
+      47.8±0.4ms          125±3ms     2.62  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 4006)
+        599±20μs      1.47±0.07ms     2.46  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 2000)
+      60.6±0.9ms          149±7ms     2.45  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 3000)
+        524±40μs      1.27±0.04ms     2.42  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 4006)
+        587±10μs      1.42±0.04ms     2.41  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 1000)
+      59.9±0.8ms        144±0.9ms     2.40  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 1000)
+        526±40μs      1.26±0.05ms     2.39  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 5000)
+        61.0±1ms          144±2ms     2.35  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 2011)
+      60.8±0.5ms        143±0.9ms     2.35  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 2000)
+      7.72±0.3μs         17.7±3μs     2.30  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 4000)
+      63.7±0.6ms          145±3ms     2.28  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 1011)
+     3.04±0.06μs         6.94±1μs     2.28  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 6000)
+        611±10μs      1.38±0.06ms     2.27  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 2011)
+        653±20μs      1.44±0.03ms     2.20  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 1011)
+     2.82±0.07μs       6.12±0.2μs     2.17  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 6000)
+     2.81±0.06μs       6.06±0.2μs     2.16  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 9000)
+     2.99±0.06μs       6.16±0.2μs     2.06  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 8000)
+     2.80±0.06μs       5.75±0.2μs     2.05  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 11000)
+     2.95±0.09μs       6.02±0.4μs     2.04  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 9000)
+     2.94±0.06μs       5.97±0.2μs     2.03  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 7000)
+        701±50μs      1.41±0.01ms     2.01  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 3000)
+      3.09±0.1μs       6.18±0.1μs     2.00  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 7000)
+      3.02±0.2μs       6.02±0.3μs     1.99  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 8000)
+      3.05±0.1μs       5.95±0.2μs     1.95  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 10000)
+      7.98±0.3μs       15.4±0.3μs     1.93  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 4006)
+      9.01±0.4μs       17.3±0.8μs     1.92  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 2011)
+      2.99±0.1μs       5.66±0.2μs     1.89  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 11000)
+     3.09±0.09μs       5.82±0.1μs     1.88  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 10000)
+      8.20±0.4μs       15.3±0.5μs     1.87  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 5000)
+      9.37±0.6μs       17.4±0.5μs     1.86  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 2000)
+      9.28±0.6μs       17.2±0.7μs     1.85  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 3000)
+      9.61±0.3μs       17.6±0.6μs     1.83  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 1011)
+      10.1±0.7μs       17.5±0.7μs     1.73  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 1000)
-      2.93±0.1μs       2.48±0.1μs     0.85  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 4000)
-     2.90±0.04μs      2.44±0.08μs     0.84  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 2011)
-     2.92±0.06μs       2.44±0.1μs     0.84  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 1011)
-      3.08±0.1μs       2.56±0.1μs     0.83  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 2011)
-      3.03±0.2μs      2.52±0.09μs     0.83  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 4006)
-      9.41±0.4μs       7.79±0.2μs     0.83  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 8000)
-      2.97±0.1μs       2.42±0.1μs     0.81  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 2000)
-      2.99±0.1μs      2.40±0.07μs     0.80  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 4006)
-      3.18±0.1μs       2.55±0.1μs     0.80  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 1000)
-      3.21±0.2μs      2.48±0.03μs     0.77  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 5000)
-      3.07±0.2μs      2.34±0.05μs     0.76  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 3000)
-      3.08±0.1μs      2.34±0.04μs     0.76  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 5000)
-      3.23±0.1μs       2.43±0.1μs     0.75  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 1000)
-      3.16±0.1μs      2.37±0.05μs     0.75  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 2000)
-      3.28±0.3μs      2.33±0.08μs     0.71  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 4000)
-      3.44±0.3μs       2.36±0.1μs     0.69  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 1011)
-        398±20μs          242±7μs     0.61  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 6000)
-        39.0±1ms       23.7±0.4ms     0.61  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 6000)
-      62.4±0.5ms       28.9±0.5ms     0.46  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 11000)
-      64.4±0.2ms       28.6±0.4ms     0.44  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 10000)
-        657±20μs         290±10μs     0.44  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 11000)
-      64.7±0.4ms       27.9±0.4ms     0.43  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 9000)
-        659±20μs          279±8μs     0.42  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 7000)
-      64.1±0.6ms       27.1±0.4ms     0.42  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 8000)
-      64.6±0.9ms       26.7±0.3ms     0.41  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 7000)
-        661±20μs          272±8μs     0.41  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 8000)
-        661±10μs         269±10μs     0.41  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 9000)
-        698±50μs          282±7μs     0.40  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 10000)
-      3.02±0.1μs         344±10ns     0.11  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 12000)
-      3.21±0.2μs         342±10ns     0.11  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 12000)
-      10.6±0.5μs         339±10ns     0.03  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 12000)
-        661±20μs          346±5ns     0.00  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 12000)
-      62.3±0.6ms         368±10ns     0.00  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 12000)
```
> can you comment on the slowdowns on the particular cases, e.g. 4000, 5000, I guess we don't care about these as much

As mentioned in the OP, i dont understand why we see slowdowns this big.  It should just be one extra `if freq >= FR_DAY` check, shouldn't make a major difference in the non-shortcircuited cases i think i might be able to fastpath some more cases, will give that a shot OK, i think i got these addressed:

```
       before           after         ratio
     [42fd7e7d]       [3fe9e107]
     <cln-nat-cmp>       <perf-periodarr_to_dt64arr>
+      2.98±0.1μs      8.88±0.08μs     2.97  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 6000)
+      3.32±0.2μs       8.17±0.7μs     2.46  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 11000)
+     3.01±0.09μs       7.33±0.3μs     2.44  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 8000)
+     3.02±0.08μs       7.32±0.3μs     2.42  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 10000)
+      2.93±0.1μs       7.07±0.2μs     2.41  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 8000)
+      3.25±0.3μs       7.43±0.2μs     2.28  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 9000)
+      3.38±0.2μs       7.70±0.6μs     2.28  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 7000)
+      3.14±0.1μs       6.98±0.2μs     2.23  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 11000)
+      3.10±0.2μs       6.68±0.2μs     2.16  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 9000)
+      3.58±0.4μs       7.34±0.3μs     2.05  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 7000)
+      3.34±0.3μs       6.76±0.2μs     2.02  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 10000)
+      3.59±0.3μs       7.07±0.2μs     1.97  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 6000)
-        39.2±1ms       23.3±0.6ms     0.59  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 6000)
-        439±30μs         239±10μs     0.54  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 6000)
-      63.1±0.5ms         30.8±3ms     0.49  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 10000)
-        649±10μs         309±30μs     0.48  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 10000)
-        65.5±2ms         30.2±3ms     0.46  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 11000)
-        64.7±2ms         29.6±3ms     0.46  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 9000)
-      62.9±0.6ms       27.1±0.7ms     0.43  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 7000)
-        64.0±1ms       26.9±0.6ms     0.42  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 8000)
-        664±20μs          267±6μs     0.40  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 7000)
-       784±100μs         308±20μs     0.39  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 11000)
-        725±90μs          282±7μs     0.39  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 9000)
-      3.16±0.2μs      1.14±0.04μs     0.36  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1, 12000)
-     3.11±0.07μs      1.11±0.04μs     0.36  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(0, 12000)
-       809±100μs          287±6μs     0.36  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 8000)
-      9.57±0.3μs      1.12±0.03μs     0.12  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(100, 12000)
-        654±20μs      1.18±0.06μs     0.00  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(10000, 12000)
-        64.4±1ms      1.18±0.04μs     0.00  tslibs.period.TimePeriodArrToDT64Arr.time_periodarray_to_dt64arr(1000000, 12000)
```

We still have some cases that see a 2-3x slowdown, but those are now all length-0 or length-1 cases where the actual time difference is tiny (and makes sense) thanks