Stabilize enum-map

[KamilaBorowska]

For 1.0.0, an Internal interface which will likely be renamed to Enum will need to be stabilized. The plan is once const generics become stable (see rust-lang/rust#44580) - although worth noting that this doesn't need full const generics, just being able to use associated consts within a struct, an Internal trait would be rewritten as such.

trait Enum {
    const DISCRIMINANTS: usize;
    fn discriminant(self) -> usize;
    fn from_discriminant(discriminant: usize) -> Self;
}

[KamilaBorowska]

Also, index_for_key function will be removed as it won't be needed after stabilizing Enum trait.

[KamilaBorowska]

It would be neat to have a compact enum map representation for bincode and similar formats, but again, incompatible change.