perf: private map partitions

[lgray]

This PR splits map_parititions into itself and an unsafe version _map_partitions which does not make the effort to sanitize input functions or data.

This avoids creating an ArgsKwargsPackedFunction when it is not necessary to do so in routine operations like getitem and fancy indexing.

Saves O(1)s in a complex analysis.

[martindurant]

I see nothing wrong with this at all. Where do we document (for ourselves) when we can call this internal method?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.10%. Comparing base (8cb8994) to head (dd319b6).
Report is 16 commits behind head on main.

Files	Patch %	Lines
src/dask_awkward/lib/core.py	96.42%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #477      +/-   ##
==========================================
+ Coverage   93.06%   93.10%   +0.03%     
==========================================
  Files          23       23              
  Lines        3290     3305      +15     
==========================================
+ Hits         3062     3077      +15     
  Misses        228      228              

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[lgray]

@martindurant added a helpful description.

[lgray]

anything blocking this one?

[martindurant]

Let's push this one in, and I'll deal with the caching PR