Move Parquet saving into a separate item list collector

[mdekstrand]

This adds ParquetItemListCollector that saves item lists as we go to a Parquet file, and does some smaller type improvements and refactors to the data code.

[codecov]

Codecov Report

❌ Patch coverage is 20.20202% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 17.74%. Comparing base (651740a) to head (dd121cc).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/data/_collection/_parquet.py	17.77%	37 Missing ⚠️
src/lenskit/batch/_runner.py	0.00%	18 Missing ⚠️
src/lenskit/batch/_results.py	0.00%	9 Missing ⚠️
src/lenskit/data/_collection/_base.py	38.46%	8 Missing ⚠️
src/lenskit/batch/_queries.py	0.00%	6 Missing ⚠️
src/lenskit/batch/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1127       +/-   ##
===========================================
- Coverage   89.55%   17.74%   -71.81%     
===========================================
  Files         256      209       -47     
  Lines       17227    14485     -2742     
===========================================
- Hits        15428     2571    -12857     
- Misses       1799    11914    +10115     

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