Add EnumMap instances

[tysonzero]

While I quite like IntMap its use often leads to primitive blindness, and really I probably want some sort of newtype over Int as the key most of the time, such as a DB key or an entity id. EnumMap seems to solve that issue, so I would like to get EnumMap up to par with IntMap on what instances it has.

[tysonzero]

Mikolaj/enummapset#14

[kozross]

@RyanGlScott Given that we depend on containers already (potentially flagged away), we could probably depend on enummapset too (and also hide it behind a flag). I feel it's a useful package (and data type), and I'd be happy to add those instances.

[RyanGlScott]

I'm generally cautious about adding new dependencies, as semigroupoids is pretty commonly depended upon. (Yes, cabal build flags exist, but in practice, most people don't finely tune their cabal flags when building packages.)

I do see the utility in the enummapset package, although I don't know if it crosses my personal threshold for incurring an extra dependency just yet. I do with something like EnumMap were part of containers, which would make this an easier pill to swallow.