PERF: DataFrame.__setitem__
Looks good based on frame_methods benchmarks, will run a full set overnight.

```
asv continuous -E virtualenv -f 1.01 master HEAD -b frame_methods
[...]
       before           after         ratio
     [0b0cac5c]       [b21496b1]
     <bug-fillna-inplace>       <perf-isetitem>
-      30.0±0.9ms       26.8±0.2ms     0.89  frame_methods.Equals.time_frame_nonunique_unequal
-        187±10ms          166±6ms     0.89  frame_methods.ToDict.time_to_dict_datetimelike('split')
-        29.9±1ms       26.6±0.2ms     0.89  frame_methods.Equals.time_frame_nonunique_equal
-        43.6±2ms         38.8±1ms     0.89  frame_methods.Isnull.time_isnull_strngs
-        39.9±2ms       35.3±0.3ms     0.89  frame_methods.Equals.time_frame_object_unequal
-        201±20ms          177±7ms     0.88  frame_methods.ToDict.time_to_dict_datetimelike('records')
-        39.6±1ms         34.5±1ms     0.87  frame_methods.Isnull.time_isnull_obj
-      5.56±0.4ms       4.81±0.3ms     0.87  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'float64')
-      5.60±0.3μs       4.84±0.2μs     0.86  frame_methods.ToDict.time_to_dict_datetimelike('series')
-        32.7±2μs       28.3±0.8μs     0.86  frame_methods.GetNumericData.time_frame_get_numeric_data
-      12.2±0.5ms       10.5±0.3ms     0.86  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'datetime64[ns, tz]')
-         238±8ms          203±6ms     0.86  frame_methods.GetDtypeCounts.time_info
-        75.4±3ms         64.6±2ms     0.86  frame_methods.Rename.time_rename_single
-     1.00±0.04ms         853±20μs     0.85  frame_methods.GetDtypeCounts.time_frame_get_dtype_counts
-      10.2±0.6ms      8.67±0.08ms     0.85  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'datetime64[ns, tz]')
-      10.1±0.6ms       8.54±0.5ms     0.85  frame_methods.Shift.time_shift(0)
-      11.7±0.9ms       9.86±0.3ms     0.85  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'datetime64[ns, tz]')
-         173±7ms          146±8ms     0.84  frame_methods.ToDict.time_to_dict_datetimelike('list')
-        74.7±3ms         63.0±1ms     0.84  frame_methods.Rename.time_rename_axis1
-        69.1±3ms         58.2±4ms     0.84  frame_methods.ToHTML.time_to_html_mixed
-      3.88±0.4ms       3.27±0.1ms     0.84  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'float32')
-        126±20ms          106±4ms     0.84  frame_methods.Reindex.time_reindex_axis1_missing
-      2.83±0.2ms      2.38±0.09ms     0.84  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'datetime64[ns]')
-      12.4±0.4ms       10.4±0.3ms     0.84  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'Int64')
-        35.1±3ms         29.4±2ms     0.84  frame_methods.SortIndexByColumns.time_frame_sort_values_by_columns
-        77.6±2ms         64.9±2ms     0.84  frame_methods.Repr.time_frame_repr_wide
-      5.37±0.7ms       4.48±0.2ms     0.83  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'float64')
-      2.98±0.2ms       2.47±0.3ms     0.83  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'timedelta64[ns]')
-        54.3±3ms         45.0±1ms     0.83  frame_methods.Equals.time_frame_object_equal
-      10.4±0.7ms       8.63±0.2ms     0.83  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'Float64')
-        451±50μs         373±20μs     0.83  frame_methods.Isnull.time_isnull
-      5.18±0.4ms       4.29±0.2ms     0.83  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'float64')
-      11.3±0.8ms       9.37±0.3ms     0.83  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'Float64')
-        83.1±3ms         68.5±2ms     0.82  frame_methods.Rename.time_dict_rename_both_axes
-      11.6±0.7ms       9.50±0.1ms     0.82  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'Int64')
-      4.46±0.5ms       3.65±0.2ms     0.82  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'float32')
-        12.3±1ms       10.0±0.5ms     0.82  frame_methods.MemoryUsage.time_memory_usage_object_dtype
-        12.2±1ms       9.95±0.3ms     0.82  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'datetime64[ns, tz]')
-        30.6±2ms         24.7±1ms     0.81  frame_methods.Reindex.time_reindex_axis1
-        534±40μs         429±30μs     0.80  frame_methods.Shift.time_shift(1)
-      7.30±0.2ms       5.86±0.2ms     0.80  frame_methods.Repr.time_html_repr_trunc_mi
-     4.81±0.07ms       3.86±0.1ms     0.80  frame_methods.Repr.time_html_repr_trunc_si
-        186±20ms          148±4ms     0.79  frame_methods.Fillna.time_frame_fillna(False, 'bfill', 'object')
-        289±60ms          229±8ms     0.79  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'object')
-      11.9±0.7ms       9.38±0.2ms     0.79  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'Int64')
-        12.2±1ms       9.63±0.8ms     0.79  frame_methods.Rank.time_rank('float')
-      11.6±0.8ms       9.16±0.2ms     0.79  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'Float64')
-        292±50ms         227±10ms     0.78  frame_methods.Fillna.time_frame_fillna(True, 'pad', 'object')
-      3.95±0.2ms       2.99±0.1ms     0.76  frame_methods.Rank.time_rank('uint')
-      4.06±0.4ms      3.07±0.09ms     0.75  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'float32')
-      4.63±0.7ms       3.47±0.4ms     0.75  frame_methods.Lookup.time_frame_fancy_lookup
-      1.82±0.2ms      1.35±0.09ms     0.74  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'datetime64[ns]')
-      4.88±0.4ms       3.62±0.3ms     0.74  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'float32')
-      5.87±0.5ms       4.33±0.2ms     0.74  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'float64')
-        193±30ms          140±7ms     0.73  frame_methods.Fillna.time_frame_fillna(False, 'pad', 'object')
-       972±100ms         700±30ms     0.72  frame_methods.Nunique.time_frame_nunique
-      4.45±0.3ms       3.17±0.2ms     0.71  frame_methods.NSort.time_nsmallest_two_columns('last')
-      5.17±0.4ms       3.66±0.3ms     0.71  frame_methods.NSort.time_nlargest_two_columns('last')
-        5.14±1ms       3.58±0.2ms     0.70  frame_methods.NSort.time_nlargest_two_columns('all')
-      1.95±0.3ms      1.15±0.06ms     0.59  frame_methods.Fillna.time_frame_fillna(True, 'bfill', 'timedelta64[ns]')
```
> pandas/core/internals/managers.py:1090: error: Argument 1 to "_iset_single" of "BlockManager" has incompatible type "Union[int, slice, ndarray[Any, Any]]"; expected "int"  [arg-type]

looks good otherwise still looking at asvs; the full run showed some slowdowns that don't make sense to me so i need to do some digging OK, the slower-looking asvs appear to be false-positives.  Good to go here i think. hmm this has a lot of test failures, let's wait to rebase till after CI patch