Add Alternative wrapper to Data.Monoid

As discussed on the libraries list,

-- | Monoid under @<|>@.
newtype Alt f a = Alt {getAlt :: f a}
  deriving (Generic, Generic1, Read, Show, Eq, Ord, Num, Enum,
            Monad, MonadPlus, Applicative, Alternative, Functor)

instance forall f a . Alternative f => Monoid (Alt f a) where
  mempty = Alt empty
  mappend = coerce ((<|>) :: f a -> f a -> f a)

The documentation for Data.Monoid.First and Data.Monoid.Last should also be expanded to explain that First a is isomorphic to Alt Maybe a and Last a is isomorphic to Dual (Alt Maybe a).

Trac metadata

Trac field	Value
Version	7.9
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
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CC	core-libraries-committee@haskell.org
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