Proposal: Add Semigroup/Monoid instance to Data.Functor.Compose

[Daniel Cartwright]

Data.Functor.Compose:

newtype Compose f g a = Compose { getCompose :: f (g a) }

there's clear-cut, more traditional instances if `f` and `g` are
Applicative:

instance (Applicative f, Applicative g, Semigroup a) => Semigroup (Compose
f g a) where
  (<>) = liftA2 (<>)

instance (Applicative f, Applicative g, Monoid a) => Monoid (Compose f g a)
where
  mempty = pure mempty

There's an alternative with `QuantifiedConstraints`, but it's arguable that
this is desirable:

instance (forall x. Semigroup x => Semigroup (f x), forall x. Semigroup x
=> Semigroup (g x), Semigroup a) => Semigroup (Compose f g a) where
  Compose x <> Compose y = Compose (x <> y)

Both to seem to fit the commonplace spirit of lifting monoids up through
applicative contexts.
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