Proposal: Add conspicuously missing Functor instances for tuples

[Ryan Scott]

WriterT (and Writer) don't quite capture what I need. With WriterT,
you have to tack on that extra m type parameter, and when you just
specialize it to Identity, it only serves as an extra hoop to jump
through to dig out the state. The prescence of the m type parameter
also prevents me from easily modifying the state with a Bifunctor
instance, whereas tuples have no such obstacle.

It's a minor difference, but an important one when I decide which data
structure to reach for.

Ryan S.