Update internal S2 version and migrate dependency build to cmake

[paleolimbot]

An experiment to use cmake instead of the wild scheme I'd used to vendor abseil + s2 before!

• [x] Compiles, links, and passes tests on MacOS
• [x] Compiles on Linux
• [ ] Links on Linux
• [ ] Passes tests on Linux
• [ ] Anything on Windows

[codecov-commenter]

Codecov Report

Merging #249 (868aff9) into main (b28fb7f) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 868aff9 differs from pull request most recent head 0227b7e. Consider uploading reports for the commit 0227b7e to get more accurate results

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   94.04%   94.04%           
=======================================
  Files          46       46           
  Lines        3524     3524           
=======================================
  Hits         3314     3314           
  Misses        210      210           

[paleolimbot]

Superceeded by #257 and #258.