Speed up ItemList indexing

[mdekstrand]

This speeds up ItemList indexing by providing a fast path through item list initialization for internal reconstruction.

[codecov]

Codecov Report

Attention: Patch coverage is 94.23077% with 6 lines in your changes missing coverage. Please review.

Project coverage is 90.14%. Comparing base (92cb429) to head (f9eb5f4).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lenskit/data/arrow.py	91.80%	5 Missing ⚠️
src/lenskit/data/items.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #754      +/-   ##
==========================================
+ Coverage   90.10%   90.14%   +0.03%     
==========================================
  Files         139      140       +1     
  Lines        9928     9995      +67     
==========================================
+ Hits         8946     9010      +64     
- Misses        982      985       +3     

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