Only traverse top-level definitions that actually have bangs

[JakobBruenker]

Since we know where the GHC throws errors about bang patterns in expression contexts, it should be possible to only traverse/modify the ASTs of the definitions those expressions are in.

Doing this should improve performance.

We could also do it in a more fine-grained way, e.g. if the bang pattern happens in a where block, there's no reason to modify anything outside of that particular definition in the where block.

[JakobBruenker]

This is probably not worth it for the time being - the performance impact in general isn't that big and #14 provides more benefit for less effort