Expose KeyPath.keys, make KeyPath Equatable

[JaviSoto]

I use this pattern to group the keys that I use in a Decodable type:

struct MyDecodableType: Decodable {
    enum Keys: KeyPath {
        case foo = "foo"
        case bar = "bar"
    }
}

I used to make those Keys enum have an associated String value, but with the switch to KeyPath, in order to be able to use KeyPath as the raw value of an enum, it needs to conform to Equatable.

[JaviSoto]

Thanks!

Do you mind rebasing this on top of beta6?

[Anviking]

beta6 should have been deleted before, master is the most up to date.