PERF/COMPAT: define platform int to np.intp
AFAIK this only affects 64 bit python on Windows.

`numpy` wants an `np.intp` (i8 on Windows) as a indexer for `take`, but pandas defines a "platform int" as a `np.int_` (i4 on Windows).  This hits performance twice, because we often start with i8, cast to i4, then numpy will cast back to i8 in its `take`.

This is an alternative to #13924 - there I explored replacing `ndarray.take` with our `take_nd` fully, but this approach solves the perf problem without much pain or risking new segfaults.

I'd still need to adjust a bunch of tests here to pass on Windows.  This is an API change for "advanced" methods like `get_indexer`, but I don't think anything is necessary beyond a doc note?

ASV:

```
    before     after       ratio
  [4a80521 ] [3ced5d5 ]
     3.49ms     2.86ms      0.82  indexing.series_take_dtindex.time_series_take_dtindex
     3.28ms     3.03ms      0.92  indexing.series_take_intindex.time_series_take_intindex
    15.16ms    14.57ms      0.96  join_merge.join_dataframe_index_single_key_bigger.time_join_dataframe_index_single_key_bigger
    19.81ms    16.62ms      0.84  join_merge.join_dataframe_index_single_key_bigger_sort.time_join_dataframe_index_single_key_bigger_sort
    14.05ms    13.63ms      0.97  join_merge.join_dataframe_index_single_key_small.time_join_dataframe_index_single_key_small
     4.83ms     4.64ms      0.96  join_merge.join_dataframe_integer_2key.time_join_dataframe_integer_2key
     1.80ms     1.63ms      0.90  join_merge.join_dataframe_integer_key.time_join_dataframe_integer_key
   633.31us   570.37us      0.90  join_merge.join_non_unique_equal.time_join_non_unique_equal
      2.39s      1.36s      0.57  join_merge.left_outer_join_index.time_left_outer_join_index
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13972?src=pr) is 85.29% (diff: 100%)

> Merging [#13972](https://codecov.io/gh/pydata/pandas/pull/13972?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **0.02%**

``` diff
@@             master     #13972   diff @@
==========================================
  Files           139        139          
  Lines         50219      50245    +26   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42819      42854    +35   
+ Misses         7400       7391     -9   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [4a80521...322b11a](https://codecov.io/gh/pydata/pandas/compare/4a805216d99b37955c97625d304980eff10cab56...322b11ac2c0d4cc6695f361dcf1e68710aae7419?src=pr)
 yes this looks reasonable. go ahead an make ready on windows. Also add an asv as above.
 pls update the docs as indicated. otherwise lgtm.
 What else were you looking for in the docs? I did add an issue ref in the first paragraph.
 ok this looks fine. can you run this on 32-bit linux as well and see that it comes up clean (IIRC you tests on windows so that should be good).
 The changes I just pushed were to get this passing on 32 bit windows.  I don't have anything handy to test 32 bit Linux, I can probably set up a VM, although I think it should generally act the same 32 bit Windows (`np.int_ == np.intp == np.int32`)
 ok I'll run on 32 bit tomorrow

I just use a macosx / vm
 This currently fails on 32-bit linux (before your PR). I recall had to do something on windows to get this to pass. Something wrong somewhere. Your PR fixes this! yeah!

```
======================================================================
FAIL: test_alignment_non_pandas (pandas.tests.frame.test_operators.TestDataFrameOperators)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/jreback/pandas/pandas/tests/frame/test_operators.py", line 1210, in test_alignment_non_pandas
    Series([1, 2, 3], index=df.index))
  File "/home/jreback/pandas/pandas/util/testing.py", line 1154, in assert_series_equal
    assert_attr_equal('dtype', left, right)
  File "/home/jreback/pandas/pandas/util/testing.py", line 878, in assert_attr_equal
    left_attr, right_attr)
  File "/home/jreback/pandas/pandas/util/testing.py", line 1018, in raise_assert_detail
    raise AssertionError(msg)
AssertionError: Attributes are different

Attribute "dtype" are different
[left]:  int32
[right]: int64
```

So as-is this passes on linux32 (and fixes the above).

lmk when good to go.
 Cool - fixed the lint issue so this should be good to go.
 thanks!
