Proposal: transformers/mtl Applicative Relaxation

[chessai .]

I think given AMP that transformers should use CPP to relax Monad
constraints to Applicative on newer versions of base.

As an example:

except :: (Monad m) => Either e a -> ExceptT e m a
except m = ExceptT (return m)

could become

except :: (Applicative m) => Either e a -> ExceptT e m a
except m = ExceptT (pure m)

Similarly some typeclasses in mtl could do with some relaxation. A related
issue: https://github.com/haskell/mtl/issues/35
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