Bump hashbrown to 0.15.2 and remove deprecated coverage_helper

[nand-nor]

This PR bumps the hashbrown dep to latest (0.15.2), addresses the deprecated use of coverage_helper::tests, and adds a lint table to remove the warning emitted for the coverage_nightly attribute. The dev dependency on coverage_helper is removed in favor of the now-recommended form:

#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
...
#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod test {
...
}

[nand-nor]

cc @sgodwincs. I notice that CI is failing but has been for awhile now, the last CI run happened long ago enough that the logs have been dropped, and it looks like this PR will need workflow approval to run CI. May be failing because code coverage is just below 95% but that is a guess. Let me know if you want CI to be passing before looking at / merging