Add -fwarn-context-quantification (#4426)

Summary:
This warning (enabled by default) reports places where a context
implicitly binds a type variable, for example

type T a = {-forall m.-} Monad m => a -> m a

Also update Haddock submodule.

Test Plan: validate

Reviewers: hvr, simonpj, goldfire, austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D211

GHC Trac Issues: #4426
Note context quantification becomes an error in 7.12; add release notes