perf: Optimise low-level null scans and arg_max for bools (when chunked)

[alexander-beedie]

Optimisations

Spotted some nice speedups for the lower-level first_not_null and last_non_null validity/bitmap scans while working on #22880; breaking them out as a standalone PR so we can take advantage while deciding on a better API for that one.

So, switching out the following offset calculations...

mask = BitMask::from_bitmap(validity) 
Some(n) = mask.nth_set_bit_idx(0, 0)               // first non-null
Some(n) = mask.nth_set_bit_idx_rev(0, mask.len())  // last non-null

...for (more or less)...

validity.leading_zeros() < validity.len()   // first non-null
validity.trailing_zeros() < validity.len()  // last non-null

...and adjusting the surrounding code slightly allows us to get the required result directly from the validity bitmap without mediating via BitMask and nth_set_bit_idx.

Speedup:

• These scans get about 3-5x faster (measured by their impact on the performance of the first/last non-null index methods in the originating PR, as compiled with build-dist-release) 🚀

Also:

• Generalised the single-chunk fast path in arg_max_bool to handle "n" chunks.
• Added some fast early-exit conditions for .index_of(None), and an associated small parametric test to improve edge-case coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.63%. Comparing base (b50a36e) to head (dc8cd62).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/utils/mod.rs	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #22897      +/-   ##
==========================================
- Coverage   80.70%   80.63%   -0.08%     
==========================================
  Files        1671     1671              
  Lines      221173   221446     +273     
  Branches     2792     2791       -1     
==========================================
+ Hits       178502   178563      +61     
- Misses      42001    42218     +217     
+ Partials      670      665       -5     

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[orlp]

Thanks :)