Add instances of monoid-subclasses?

[blamario]

Since I fixed issue #24 of monoid-subclasses, the containers' Map and IntMap instances have become less useful. I considered adding instances for MonoidalMap in their place, but monoidal-containers has become a heavy dependency since its addition of aeson.

As an alternative, I wonder if you'd be open to a PR adding the monoid-subclasses as a dependency and the relevant instances? It would bring in only one transitive dependency, namely primes.

[fumieval]

-1 from me. monoid-subclasses seems to be a lot more progressive than monoidal-containers where its sole purpose is to provide a different definition of the Semigroup instance, so I think it should be the other way around.

I believe the plan is to switch to indexed-traversable, which reduces the dependency footprint a lot