Add Rust acceleration to User KNN

[mdekstrand]

This rewrites User KNN scoring in Rust, and uses SciPy instead of Torch for similarities. This also removes the last need for some of the Torch sparse matrix normalization code.

Closes #718.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.35%. Comparing base (868d3aa) to head (a334288).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #730      +/-   ##
==========================================
- Coverage   90.36%   90.35%   -0.01%     
==========================================
  Files         137      137              
  Lines        9741     9694      -47     
==========================================
- Hits         8802     8759      -43     
+ Misses        939      935       -4     

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