Proposal: Foldable typeclass: make foldl' and foldr' class methods

[Paterson, Ross]

It's a pity to have to add methods to get around GHC's limitations,
but it seems harmless.  Looks like you'll want strict versions of all
5 methods, though.

> The only potential breakage is that foldl' and foldr' are exported via
> Foldable(..) rather than directly. This could affect modules that use
> explicit imports.

I don't think so.  You can import methods separately.