Speed up stimes in instance Semigroup Endo
Rebased and squashed what's left from !6863 (closed). CC @treeowl
Implements https://github.com/haskell/core-libraries-committee/issues/4
Rebased and squashed what's left from !6863 (closed). CC @treeowl
Implements https://github.com/haskell/core-libraries-committee/issues/4