Setup codecov

[CathalMullan]

Hopefully less flakey now. I picked a bad day to try it out last time: https://status.codecov.com/incidents/1g1pznmcxr8t

Far too many Nix shells, but 🤷

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #120 will not alter performance

_{Comparing 87-set-up-codecov-in-ci (bff06de) with main (96fb53f)}

Summary

✅ 16 untouched benchmarks

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:white_check_mark: All tests successful. No failed tests found.

see 6 files with indirect coverage changes