Enum concept

Boost.Hana currently has `Struct`. I have some code that performs automatic serialization for `Struct` types. However an `Enum` is missing in Boost.Hana to help here further.

> The `Struct` concept [...] it allows accessing the names of the members of a user-defined type, and also the value of those members

For the `Enum` concept, I'd expect a similar mapping, however values should be `hana::int_c` (non-mutable).

Start by reading Boost.Hana's existing Struct concept documentation and implementation, then trace the related tests or examples if available. Clarify how an Enum should expose its mapping and how non-mutable hana::int_c values are represented; done means the concept is documented, implemented, and covered by tests.

Written by the indexing model from the issue text.