Add instances for `MonadGen`

[garyb]

If we add MonadGen instances then Jack can easily be swapped with QC or SC for types that provide generators in terms of MonadGen.

This also means libraries using jack for testing can provide generators for their types without promoting it from a devDependency to a dependency.

[jacobstanley]

Sounds good to me. It has drawn my attention to something a little annoying though, I had planned to do a purescript version of hedgehog at some point, and it already has a MonadGen class which serves a similar purpose.