yesod-form fails to build with transformers >= 0.6

[TeofilC]

In transformers-0.6, the MonadTrans class was changed to have a quantified constraint:

class (forall m. Monad m => Monad (t m)) => MonadTrans t where
...

This causes the MonadTrans AForm instance to fail.

I feel like there's two solutions:

1. Add a Monad m => Monad (AForm m) instance for AForm though I feel like there's probably a reason for a lack of one
2. Remove the MonadTrans instance

[snoyberg]

I think (1) makes sense, but gated via CPP on the version of transformers

[TeofilC]

Sure I'll write up a patch next week.