[CLN] Merge inference into lib.pyx
Continue trimming down _libs/src.

src/inference is a minor hassle, particularly because ATM there are imports/cimports in both it and lib.  Its existence also complicates setup.py a bit.

Evidently parse_helper.h depended on the order in which it was included (I assume this is a code smell in C, but who knows).  Fixed this.

This is almost cut/paste.  The changes are:
 - Arrange (and de-duplicate) imports at the top of lib
 - whitespace fixups
 - change `for i from 0 <= i < n:` to `for i in range(n):` in a few places
 - a couple of comments about potential optimizations in the next pass

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22219?src=pr&el=h1) Report
> Merging [#22219](https://codecov.io/gh/pandas-dev/pandas/pull/22219?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/00416816fa5be34567185149fdfcbf2d0e068af0?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22219/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22219?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22219      +/-   ##
==========================================
+ Coverage   92.06%   92.06%   +<.01%     
==========================================
  Files         169      169              
  Lines       50698    50698              
==========================================
+ Hits        46675    46676       +1     
+ Misses       4023     4022       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22219?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22219/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <0%> (+0.05%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22219?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/22219?src=pr&el=footer). Last update [0041681...6d33432](https://codecov.io/gh/pandas-dev/pandas/pull/22219?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > why is this a good idea? i thought we discussed a long time ago to make this a separate module
isn’t that better than making s bigger module? or is this just not possible ?

Think of these as orthogonal goals.  This doesn't change the size of the module, just un-splits the file.  ATM I find myself switching between files to make sure I know what's in the namespace, which really doesn't have any upside.  Plus its progress towards killing off src/

There are a few more things we can move out; most of the low-hanging fruit has been picked. ok this is fine, ideally can pull out a separate inference module that is orthogonal though.