HLint everything and add to CI

[siddharth-krishna]

Spring cleaning: thought I'd get rid of all the blue squiggles while there weren't many open PRs. :)

I ran:

cabal install apply-refact hlint
for f in `git ls-files | grep -e '.*\.hs'`; do echo linting $f; hlint --refactor --refactor-options="-i" $f; done

And manually checked that nothing semantically changed.

Also trying out a GitHub action that should generate lint suggestions in the PRs.
I thought the action would add PR suggestions in the web interface which could be applied with one click, but it didn't do that, and going through the CI logs feels a bit painful. This other action might do that, but it needs some security/token set up so I'll save it for another time:
https://github.com/haskell-actions/hlint-scan

For now, it just adds a hint to the PR diff view:

[albertov]

🧼