Custom derive

[Michael-F-Bryan]

Now that custom derives are stable, would this be better done using them instead of a macro_rules macro? That way you can have the FromPrimitive and ToPrimitive trait definitions in enum_primitive and then a enum_primitive_derive crate to do the deriving.

I think something like this is more applicable to macros 1.1 than macros by example, and that way you also fix up long-standing issues like being unable to document each variant.

[cardoe]

fwiw, I've got one published at https://crates.io/crates/enum-primitive-derive