Semigroup and Monoid instances for ST

In base-4.9, a Monoid instance for IO was introduced. I would like to propose adding a similar instance for ST. This should be even less controversial since there is only one meaningful Monoid instance for ST (it lacks the exception-catching facilities of IO). The behavior of the Semigroup and Monoid instances would need to match this:

instance Monoid a => Monoid (ST s a) where
  mappend = liftA2 mappend
  mempty = pure mempty

This would let me use fold and foldMap over ST computations, which would occasionally be useful.

Trac metadata

Trac field	Value
Version	8.2.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
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