Prepare for 2.0 release

[garyb]

This re-exports the more common stuff out of Foldable and Traversable. Anything else we want to include?

Resolves #72, resolves #70

As for #71, I guess we can do that post-release, since it shouldn't affect the interface.

[paf31]

do we want to limit foldM to MonadRec?

I would say people should use FreeT to make things stack-safe first, but there is a valid use case where that might be overkill: using foldM with List to generate a graph. Indeed, I use that in the book, and I wouldn't want to have to drag in FreeT there (which probably means it is too advanced). So I think yes, let's add this as a separate function for now.

[garyb]

There's also the point raised in purescript/purescript#2359 about the not insignificant overhead if you don't need it too. I'll add it as a separate thing then. foldMRec good for the name?

[garyb]

I went with foldRecM to match tailRecM, etc.