Stable Sorting Algorithm for Fillna Indexer
- [X] closes #21207
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Haven't added a whatsnew yet if only because I need to think through the best way to phrase this / investigate where it doesn't work. I have a feeling the default `kind` argument has a bug in it back in NumPy, but `mergesort` is guaranteed as a stable sort so may be preferable anyway in spite of the performance hit.

https://docs.scipy.org/doc/numpy/reference/generated/numpy.sort.html#numpy.sort

Will post ASVs later as well - let me know of feedback in the interim
Hello @WillAyd! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on May 25, 2018 at 23:18 Hours UTC On review I don't think this is a bug with NumPy - the default quicksort implementation is just not stable (and documented as such), but the code here assumes it. Learned something new today...

FWIW there are a few instances in `hashtable.pyx` and `index.pyx` which use the default quicksort implementation. Not saying these are an issue but may be worth closer review if we notice strange behavior elsewhere # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=h1) Report
> Merging [#21212](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6abb6148c8dc14e6279f31f2e620ecf52822107?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21212/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21212   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (ø)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=footer). Last update [f6abb61...617ae08](https://codecov.io/gh/pandas-dev/pandas/pull/21212?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 ASVs below - I obviously didn't improve anything here but seems in line with all other methods

```bash
       before           after         ratio
     [dc02831f]       [f6abb614]
-         418±7μs          380±5μs     0.91  groupby.GroupByMethods.time_dtype_as_field('int', 'prod', 'transformation')
-        455±10μs          410±7μs     0.90  groupby.GroupByMethods.time_dtype_as_group('float', 'head', 'direct')
-         180±3μs          162±6μs     0.90  groupby.GroupByMethods.time_dtype_as_field('int', 'shift', 'direct')
-       158±0.6μs          141±4μs     0.89  groupby.GroupByMethods.time_dtype_as_field('float', 'prod', 'direct')
-         251±8μs          221±4μs     0.88  groupby.GroupByMethods.time_dtype_as_group('float', 'first', 'transformation')
-         471±3μs          414±5μs     0.88  groupby.GroupByMethods.time_dtype_as_field('float', 'nunique', 'transformation')
-         210±3μs          183±9μs     0.88  groupby.GroupByMethods.time_dtype_as_field('int', 'last', 'direct')
-        99.2±2μs       86.3±0.7μs     0.87  groupby.GroupByMethods.time_dtype_as_field('float', 'any', 'direct')
-        251±10μs          217±5μs     0.87  groupby.GroupByMethods.time_dtype_as_group('float', 'first', 'direct')
-         285±8μs          246±1μs     0.86  groupby.GroupByMethods.time_dtype_as_field('float', 'bfill', 'transformation')
-         287±3μs        246±0.8μs     0.86  groupby.GroupByMethods.time_dtype_as_field('float', 'bfill', 'direct')
-         229±4μs          196±7μs     0.85  groupby.GroupByMethods.time_dtype_as_field('int', 'max', 'direct')
-         169±4μs         143±10μs     0.85  groupby.GroupByMethods.time_dtype_as_field('float', 'max', 'transformation')
-         150±1μs         126±10μs     0.84  groupby.GroupByMethods.time_dtype_as_field('float', 'last', 'transformation')
-         184±2ms        154±0.6ms     0.84  groupby.GroupByMethods.time_dtype_as_field('int', 'skew', 'direct')
-         101±1μs       84.1±0.3μs     0.83  groupby.GroupByMethods.time_dtype_as_field('float', 'any', 'transformation')
-         475±9μs         396±20μs     0.83  groupby.GroupByMethods.time_dtype_as_field('float', 'nunique', 'direct')
-         253±1μs          211±1μs     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'var', 'direct')
-        99.0±4μs       82.4±0.8μs     0.83  groupby.GroupByMethods.time_dtype_as_field('float', 'all', 'transformation')
-           603ms       501±0.03ms     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'mad', 'transformation')
-         622±7μs         515±30μs     0.83  groupby.GroupByMethods.time_dtype_as_field('float', 'cummin', 'direct')
-         225±5μs          186±3μs     0.83  groupby.GroupByMethods.time_dtype_as_field('int', 'min', 'transformation')
-       140±0.9ms        115±0.8ms     0.82  groupby.GroupByMethods.time_dtype_as_field('int', 'unique', 'direct')
-        464±10μs          380±3μs     0.82  groupby.GroupByMethods.time_dtype_as_field('int', 'tail', 'transformation')
-         130±1μs          106±8μs     0.82  groupby.GroupByMethods.time_dtype_as_field('int', 'count', 'transformation')
-         100±1μs       81.2±0.2μs     0.81  groupby.GroupByMethods.time_dtype_as_field('float', 'all', 'direct')
-     1.23±0.01ms        1000±10μs     0.81  groupby.GroupByMethods.time_dtype_as_field('int', 'value_counts', 'transformation')
-         237±5μs          191±3μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'max', 'transformation')
-         235±3μs          189±3μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'min', 'direct')
-         360±4μs          289±4μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'ffill', 'transformation')
-         257±2μs          206±2μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'var', 'transformation')
-         330±5μs          265±3μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'std', 'direct')
-         189±5ms          151±2ms     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'skew', 'transformation')
-        485±20μs         388±20μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'nunique', 'transformation')
-         438±4μs          349±1μs     0.80  groupby.GroupByMethods.time_dtype_as_field('int', 'sum', 'direct')
-         100±2μs       79.6±0.8μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'all', 'transformation')
-        95.6±3μs         75.9±5μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'all', 'direct')
-         141±3ms        112±0.6ms     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'unique', 'transformation')
-           603ms        478±0.4ms     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'mad', 'direct')
-     1.26±0.03ms          997±5μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'value_counts', 'direct')
-        447±20μs          351±2μs     0.79  groupby.GroupByMethods.time_dtype_as_field('int', 'sum', 'transformation')
-         210±3μs          164±9μs     0.78  groupby.GroupByMethods.time_dtype_as_field('int', 'last', 'transformation')
-         168±5μs          131±4μs     0.78  groupby.GroupByMethods.time_dtype_as_field('float', 'min', 'direct')
-        477±10μs          371±4μs     0.78  groupby.GroupByMethods.time_dtype_as_field('int', 'median', 'direct')
-        93.1±1μs         72.3±4μs     0.78  groupby.GroupByMethods.time_dtype_as_field('int', 'size', 'direct')
-         335±2μs          260±3μs     0.77  groupby.GroupByMethods.time_dtype_as_field('int', 'std', 'transformation')
-        736±10μs         565±10μs     0.77  groupby.GroupByMethods.time_dtype_as_field('int', 'cumsum', 'direct')
-         200±2μs          153±2μs     0.77  groupby.GroupByMethods.time_dtype_as_field('float', 'median', 'direct')
-         295±4μs          225±1μs     0.76  groupby.GroupByMethods.time_dtype_as_field('float', 'ffill', 'direct')
-         281±5μs          213±1μs     0.76  groupby.GroupByMethods.time_dtype_as_field('float', 'ffill', 'transformation')
-         161±1μs          122±7μs     0.76  groupby.GroupByMethods.time_dtype_as_field('float', 'prod', 'transformation')
-         358±4μs          270±3μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'ffill', 'direct')
-         400±8μs          301±2μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'mean', 'direct')
-         486±5μs          365±2μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'nunique', 'direct')
-        922±20μs         690±10μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'cumprod', 'direct')
-        471±10μs          351±5μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'median', 'transformation')
-     1.03±0.02ms          769±7μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'rank', 'direct')
-         131±2μs         97.7±4μs     0.75  groupby.GroupByMethods.time_dtype_as_field('int', 'count', 'direct')
-        719±20μs          535±2μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'cumsum', 'transformation')
-     1.02±0.01ms          759±6μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'rank', 'transformation')
-         269±7μs        200±0.8μs     0.74  groupby.GroupByMethods.time_dtype_as_field('float', 'cumcount', 'transformation')
-         917±7μs          680±3μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'cumprod', 'transformation')
-         352±2μs        260±0.5μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'bfill', 'direct')
-         261±8μs        193±0.6μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'cumcount', 'transformation')
-         358±2μs          264±6μs     0.74  groupby.GroupByMethods.time_dtype_as_field('int', 'bfill', 'transformation')
-         156±6μs          115±2μs     0.73  groupby.GroupByMethods.time_dtype_as_field('float', 'mean', 'transformation')
-        634±10μs          465±2μs     0.73  groupby.GroupByMethods.time_dtype_as_field('float', 'cumprod', 'transformation')
-        676±20μs         490±10μs     0.73  groupby.GroupByMethods.time_dtype_as_field('float', 'cumsum', 'direct')
-         224±1μs          163±1μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'first', 'direct')
-        741±10μs         536±10μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'cummin', 'transformation')
-         222±3μs        160±0.7μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'first', 'transformation')
-         102±2μs       73.4±0.2μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'any', 'direct')
-        644±20μs          465±2μs     0.72  groupby.GroupByMethods.time_dtype_as_field('float', 'cumsum', 'transformation')
-         411±4μs          296±2μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'mean', 'transformation')
-         150±3μs        108±0.3μs     0.72  groupby.GroupByMethods.time_dtype_as_field('float', 'last', 'direct')
-       103±0.6μs       73.7±0.8μs     0.72  groupby.GroupByMethods.time_dtype_as_field('int', 'any', 'transformation')
-           595ms          424±4ms     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'mad', 'direct')
-        713±10μs         508±30μs     0.71  groupby.GroupByMethods.time_dtype_as_field('int', 'cummax', 'direct')
-         162±1μs        115±0.7μs     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'first', 'direct')
-           1.84s            1.30s     0.71  groupby.GroupByMethods.time_dtype_as_field('int', 'describe', 'transformation')
-         184±3μs          130±2μs     0.71  groupby.GroupByMethods.time_dtype_as_field('int', 'shift', 'transformation')
-           1.83s            1.29s     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'describe', 'transformation')
-         162±4μs          115±1μs     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'first', 'transformation')
-        637±10μs          450±6μs     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'cummax', 'direct')
-        666±10μs          470±5μs     0.71  groupby.GroupByMethods.time_dtype_as_field('float', 'cumprod', 'direct')
-         273±4μs          192±2μs     0.70  groupby.GroupByMethods.time_dtype_as_field('int', 'cumcount', 'direct')
-           1.84s            1.29s     0.70  groupby.GroupByMethods.time_dtype_as_field('int', 'describe', 'direct')
-        476±10μs        333±0.6μs     0.70  groupby.GroupByMethods.time_dtype_as_field('int', 'head', 'direct')
-         447±3μs        311±0.8μs     0.70  groupby.GroupByMethods.time_dtype_as_field('float', 'head', 'transformation')
-        456±10μs          315±1μs     0.69  groupby.GroupByMethods.time_dtype_as_field('float', 'head', 'direct')
-           600ms          414±2ms     0.69  groupby.GroupByMethods.time_dtype_as_field('float', 'mad', 'transformation')
-           1.85s            1.27s     0.69  groupby.GroupByMethods.time_dtype_as_field('float', 'describe', 'direct')
-         166±2μs          113±2μs     0.68  groupby.GroupByMethods.time_dtype_as_field('float', 'mean', 'direct')
-         214±2μs          143±1μs     0.67  groupby.GroupByMethods.time_dtype_as_field('float', 'median', 'transformation')
-     1.08±0.08ms         565±60μs     0.52  groupby.GroupByMethods.time_dtype_as_field('int', 'sem', 'direct')
-      1.35±0.2ms          605±8μs     0.45  groupby.GroupByMethods.time_dtype_as_field('int', 'pct_change', 'transformation')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
``` thanks @WillAyd 