Export `traverse` and `Foldable` methods

[L0neGamer]

I was doing some cleanup in haskell/transformers#115, and realised that there is some disconnect between GHC's Prelude-exported types and MicroHs's.

MicroHs exports Foldable and Traversable but not their methods. It would be useful for the methods to be exported along with the class.

This presents an issue with regards to the foldable methods which share names with a bunch of list monomorphised functions. For the most part this should be fine, but it's worth mentioning.

[claudeha]

e.g. tidal-mondo package expects traverse in Prelude, workround is patch it to add import Data.Traversable (traverse)

[augustss]

It's very intentional that the Foldable and Traverseable methods are not exported. I don't like the Prelude change that exports those, so MicroHs does not do that. That said, traverse and sequenceA do not clash with anything, so I have exported those.

[L0neGamer]

Not that I'd want you to make changes purely for my own codebase's cleanliness, but would foldMap be a suitable export as well?

I don't think it would make sense to export exactly one method from Foldable, so while I think it would be convenient to export just the Foldable methods I accept that it isn't what you want for MicroHs.

[augustss]

Actually, I don't mind exporting foldMap since it does not clash with anything in Data.List.
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