Add Cython build infra + initial extension module

[mdekstrand]

This adds support for building extension modules with Cython, and uses it to implement a Cython-based kvp. This lays the groundwork for remaining work for #338.

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 91.05%. Comparing base (2580591) to head (daab9c3).
Report is 10 commits behind head on main.

❗ Current head daab9c3 differs from pull request most recent head a9b39a1. Consider uploading reports for the commit a9b39a1 to get more accurate results

Files	Patch %	Lines
lenskit/util/kvp.pyx	92.85%	5 Missing ⚠️
lenskit/util/csmatrix.pyx	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
+ Coverage   91.00%   91.05%   +0.04%     
==========================================
  Files          40       42       +2     
  Lines        3125     3209      +84     
==========================================
+ Hits         2844     2922      +78     
- Misses        281      287       +6     

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