workflow: add codecov

[KevFan]

• Add coverage
  • Test and generate code coverage using https://github.com/taiki-e/cargo-llvm-cov

Closes: #169

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@775596d). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #192   +/-   ##
=======================================
  Coverage        ?   73.46%           
=======================================
  Files           ?       29           
  Lines           ?     4786           
  Branches        ?        0           
=======================================
  Hits            ?     3516           
  Misses          ?     1270           
  Partials        ?        0           

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[alexsnaps]

Let's keep this as is for now, but if we get some issues with the tests with nightly (we shouldn't but... famous last words) - we may want to run tests on stable (without coverage) too. Also, leaving this for reference...