Replace EnumArray by generic associated type

[phimuemue]

I read in README that

library doesn't provide Minimum Supported Rust Version (MSRV).

If this means that we could use features from stable Rust, here's a PR that eliminates EnumArray, exploiting GADTs. This allows simpler trait bounds (simply Enum instead of EnumArray<V>), which may be valuable for users.

[sugar700]

This is something I already look into in gat branch https://github.com/xfix/enum-map/tree/gat. At this point I'm mostly trying to figure out what do I do about LENGTH in trait and whether this is a sound approach. This is a breaking change, so it's something I want to get right.

MSRV isn't an issue, even if enum-map had an actual MSRV policy (and I will probably provide one once some policy gets determined in rust-lang/libs-team#72), using GATs would be a breaking change anyways (removal of EnumArray means that bounds that used it no longer work), so it would have to be released as enum-map 3.0.0.

[DCNick3]

I really love the change! It allows me to make a struct generic over EnumMap key without exposing the values I want to use inside. I would really like to see this moving forward!

What potential soundness problems do you see?

[sugar700]

Closing, the current pull request is #35.