Show code coverage of haskell diffs on PRs in CI

I think it would be really helpful to be able to see clearly the code coverage of a set of changes in a PR as a way of validating tests and making it easier to have confidence in PRs in code reviews.

I only have an idea for how to do this with the haskell code:

convert tix to lcov with: https://hackage.haskell.org/package/hpc-lcov
...then use something like https://github.com/Connected-Information-systems/diff-test-coverage

it would be nice if this was right up front, so maybe write a github bot to post it right in the thread?