refactor ~ improve code coverage implementation

[rivy]

• modifies GHA:CICD workflow to successfully calculate code coverage for macos, unix, and windows
• uploads coverage maps to CodeCov.io (which can integrate reports from multiple platforms)
• includes shortcut utilities to calculate and show locally generated code coverage

[rivy]

@sylvestre , if you have some time, would you look at the "util/show-codecov.sh" shell script?

Specifically, I've marked three lines "FixME".

It's probably just a brain misfire, but I can't figure out how to code this to condense the ignore's for grcov into a variable and use it at the later two spots as an option to grcov. When I've tried to debug using echo ..., the command line seems correct, but grcov ignores the options included via ${GRCOV_IGNORE_OPTION}. And, so, I'm having to duplicate the ignores, writing them directly on the commands lines.
😕

[rivy]

As a reminder, I'll update the GHA:CICD workflow to include some appropriate grcov line and branch exclusions (eg, --excl-br-line 'assert(_eq|_ne)?!') when mozilla/grcov#416 is merged.

[sylvestre]

Yeah, bravo, thanks :)