[Proposal] Add default implementation of mappend/mempty in terms of mconcat

Hello, I noticed that the current Monoid class doesn't actually provide default implementations of mappend/mempty in terms of mconcat, even though this is technically very simple:

mempty = mconcat [] mappend a b = mconcat [a, b]

This would be a rather small and non-invasive change that shouldn't break any existing programs. The main downside is that an empty Monoid declaration would produce no warnings, but #7633 gives us the ability to solve this. Discussion period: 2 weeks