Fix incompatibility with llvm-cov

[Finomnis]

Rationale

Coverage generated by llvm-cov frequently report #[traced_test] as uncovered. (example)

Reason is that the generated code in call_once only gets executed once per project, and hence all tests (except one) report the code inside of it as uncovered.
Another reason is that the functions logs_contain and logs_assert do not necessarily have to be used by the programmer.

Solution

• Move call_once into the non-macro part of the library, and call it from the macro part.
• Add feature llvm-cov-compat which marks logs_contain and logs_assert as:

  #[cfg_attr(coverage_nightly, coverage(off))]

Todo:

• Confirm that this change fixes the coverage issue. (example)

[Finomnis]

Closed due to inactivity.