Add codecov and other badges/checks we have in Thicket

[pearce8]

No description provided.

[ilumsden]

@pearce8 before I can start on Codecov stuff, I need to get a sense of what tests you want factored into the reporting. It would be easy to get coverage for unit tests (run by benchpark unit-test). I think it would be possible to get coverage for all the other tests in CI, but I'm less confident about that.

[pearce8]

@ilumsden Let's start with the unit tests coverage. I think the CI tests will need to be treated separately.

[ilumsden]

@pearce8 I just created #582 as a WIP PR for this