Allow custom encoding functions for RawRepresentable enums

[tengyifei]

Suppose the following:

enum Gender: Int {
    case female
    case male
}

extension Gender: CerealType {
    // custom encode and decode
}

And encode was called on a variable of type Gender, then the RawRepresentable override version will be called, ignoring the custom serializers.

I know it's not a huge issue, but it could be fixed by providing an encodeCereal function for example, to explicitly invoke the custom serializers.