Hook up code coverage reporting to CI

[MonsieurNicolas]

What problem does your feature solve?

Rust added better support for code coverage last year.

We should take advantage of this.

What would you like to see?

At a minimum, reports per build on code coverage stats to help reviewers & to help prioritize code cleanup/testing.

Better:

• in the context of a PR, provide "diffs" on the coverage point of view, to easily spot when there might be gaps in coverage

What alternatives are there?

It may be possible to have some sort of report on a schedule that updates a static page on the project (either as a GH action, or from Jenkins).