Lazier Semigroup instance for Maybe

Mailing list discussion here: https://mail.haskell.org/pipermail/libraries/2018-May/028818.html

The existing Semigroup instance for Maybe is:

instance Semigroup a => Semigroup (Maybe a) where
    Nothing <> b       = b
    a       <> Nothing = a
    Just a  <> Just b  = Just (a <> b)

It has been proposed that it be replaced by:

instance Semigroup a => Semigroup (Maybe a) where
    Nothing <> b = b
    Just a  <> b = Just (maybe a (a<>) b)

This is lazier in the second argument, making it more consistent with the strictness of the Semigroup instances for And,Or,Ord,Either,Proxy and (). Equivalently, we could write:

instance Semigroup a => Semigroup (Maybe a) where
    Nothing <> b = b
    Just a  <> Nothing = Just a
    Just a  <> Just b  = Just (a <> b)

This makes it a little more clear that we aren't building a closure for partial function application, but it should have the same behavior.

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