Add native Parquet load/save to item list collections

[mdekstrand]

This adds support for loading and saving item list collections in a native Parquet format that preserves empty lists and is noticeably faster to load.

It also forces ItemList scores to be single-precision floating-point.

[codecov]

Codecov Report

Attention: Patch coverage is 87.15596% with 28 lines in your changes missing coverage. Please review.

Project coverage is 90.18%. Comparing base (5c830c6) to head (694b312).
Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
lenskit/lenskit/data/items.py	83.47%	20 Missing ⚠️
lenskit/lenskit/data/collection.py	92.00%	4 Missing ⚠️
lenskit/lenskit/data/mtarray.py	92.50%	3 Missing ⚠️
lenskit/lenskit/data/checks.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
- Coverage   90.31%   90.18%   -0.14%     
==========================================
  Files         100      100              
  Lines        5950     6145     +195     
==========================================
+ Hits         5374     5542     +168     
- Misses        576      603      +27     

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