Move to using cargo llvm-cov for coverage

[alexdewar]

Currently, we're using tarpaulin to generate our test coverage. The problem is that it seems to falsely report lots of lines as not being covered (see upstream issue), which is not particularly helpful.

It seems that cargo-llvm-cov is a better, more modern alternative. I gave it a whirl and the coverage seemed to be much better. I won't give the full stats, but it gives about ~95% coverage overall cf. ~85% for tarpaulin (and fixes some of the false negatives I saw), so it seems that it's more reliable.

To do:

• Change the cargo-build-and-test.yml CI workflow to generate coverage using cargo-llvm-cov cf. tarpaulin and upload it to codecov
• Add a description on how to use it locally to the developer documentation