Relax constraints on Semigroup and Monoid instances

[sjakobi]

No description provided.

[treeowl]

This is a breaking change, through it's unlikely to break much if any real code.

data T a = T deriving (Eq, Generic)
instance a ~ [Int] => Hashable (T a)

buh :: HashMap (T a) ()
buh = empty 

asHTof :: a -> HashMap (T a) x -> a
asHTof a _ = a

foo :: Int
foo = length (mempty `asHTof` whatever)
  where
    !whatever = buh <> buh

If you remove the Hashable constraints, this code will no longer compile.

It's possible to construct even worse scenarios using defaulting, in which it compiles both ways but with different results.

[treeowl]

I'm only mentioning that because it needs a major version bump along with a change log entry.