Require raw values in enum

[jpsim]

The opposite of our current rule that triggers a violation if the same string values as enum member names were used, resulting in redundant duplication, would sometimes be useful if you want to allow refactoring of the Swift API without breaking the API contract.

[marcelofabri]

This should be a good rule for someone wanting to begin contributing to SwiftLint. RedundantStringEnumValueRule can be used as an inspiration (but I'd expect this one to be actually simpler).

I also think this should be opt-in.

[ornithocoder]

There's also the Error case to think about.

enum FooError: Error {
    case expected(number: Int, got: Int)
}

[jpsim]

Yes, definitely opt-in.