Make enum keys of type EnumKey, not int

[jkomoros]

Currently keys in Enums are all ints.

It's probably more clear to make them type EnumKey int, just for clarity. For example, GameDelegate.GroupMembership has to explain in documentation what map[int]bool means.

It's a little confusing that a given int isn't guaranteed to be in a given Enum, but at least it's clear what the intent is.