Add enum support.

[itdaniher]

It'd be nice to be able to make use of enums - it's a feature I used with capnp that was quite helpful for weird datastructure shenanigans. Opening an issue to collect thoughts before I give it a stab.

[itdaniher]

Coercion to and from uint would be useful for my purposes, largely just as syntactic sugar. Turns out msgpack doesn't define an enum type.

[mneumann]

There is a new msgpack spec which includes user-defined types. But this will never be interoperable with other languages, so it's better not to use.

[mneumann]

Yeah, it's not interoperable.