Use cargo llvm-cov for coverage reports

[luizirber]

Explore using https://github.com/taiki-e/cargo-llvm-cov for Rust coverage report

I think this might unlock Python+Rust coverage too, where we measure coverage for Rust code from python tests too.
More info:

• https://github.com/cjermain/rust-python-coverage
• ci: add coverage for pytests PyO3/pyo3#2105

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.91%. Comparing base (f3dc5f2) to head (b472e7b).
Report is 366 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #2993      +/-   ##
==========================================
- Coverage   86.63%   82.91%   -3.73%     
==========================================
  Files         135      135              
  Lines       15389    20274    +4885     
  Branches     2624    10371    +7747     
==========================================
+ Hits        13333    16810    +3477     
- Misses       1755     2938    +1183     
- Partials      301      526     +225     

Flag	Coverage Δ
hypothesis-py	25.56% <ø> (ø)
python	92.85% <ø> (ø)
rust	66.83% <ø> (+7.40%)	⬆️

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