Remove CSR class in favor of separate library

[mdekstrand]

This makes LensKit use the external CSR library instead of carrying the CSR code around in our source tree.

[codecov]

Codecov Report

Merging #218 (eb324ec) into main (2d13052) will increase coverage by 0.18%.
The diff coverage is 95.79%.

@@            Coverage Diff             @@
##             main     #218      +/-   ##
==========================================
+ Coverage   92.42%   92.61%   +0.18%     
==========================================
  Files          43       41       -2     
  Lines        3987     3451     -536     
==========================================
- Hits         3685     3196     -489     
+ Misses        302      255      -47     

Impacted Files	Coverage Δ
lenskit/algorithms/tf/__init__.py	75.00% <0.00%> (-6.82%)	⬇️
lenskit/algorithms/tf/bpr.py	89.43% <75.00%> (+0.07%)	⬆️
lenskit/data.py	93.33% <93.33%> (ø)
lenskit/algorithms/item_knn.py	84.02% <98.18%> (+3.03%)	⬆️
lenskit/algorithms/als.py	97.76% <100.00%> (+0.01%)	⬆️
lenskit/algorithms/basic.py	93.08% <100.00%> (ø)
lenskit/algorithms/implicit.py	82.60% <100.00%> (ø)
lenskit/algorithms/svd.py	94.64% <100.00%> (ø)
lenskit/algorithms/user_knn.py	97.58% <100.00%> (-0.17%)	⬇️
lenskit/util/test.py	87.50% <100.00%> (+0.46%)	⬆️
... and 3 more

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