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 17.83%. Comparing base (a3282e8) to head (b41a6e1).

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   89.53%   17.83%   -71.71%     
===========================================
  Files         256      209       -47     
  Lines       17167    14419     -2748     
===========================================
- Hits        15370     2571    -12799     
- Misses       1797    11848    +10051     

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