Set up formatting in CI

[nikita-volkov]

Strategy

I see two options:

1. Checking. Fail on incorrect formatting.

2. Autoformatting as a chore job triggered by commits on master.

I vote for 2 (it served me well in my practice), but 1 is more common.

Formatters

For Haskell I suggest to just go with Ormolu since it's minimal amount of decisions.

For Cabal I suggest to go with cabal-gild.

[winitzki]

Did the auto-formatting chore job ever fail in your experience?

[nikita-volkov]

Quite rarely. Primarily due to bugs in the formatter (which have since been fixed).