[CLN] Merge inference into lib.pyx

[jbrockmendel]

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]

Codecov Report

Merging #22219 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	42.32% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/base.py	96.43% <0%> (+0.05%)	⬆️

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[jreback]

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 ?

[jbrockmendel]

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.

[jreback]

ok this is fine, ideally can pull out a separate inference module that is orthogonal though.