Allow customization of generated enum/property name

[nevillco]

The macro in the example generates:

enum StaticMemberSwitchable {
    case foo
    case bar
    case baz
}
var switchable: StaticMemberSwitchable {
    switch id {
        case Self.foo.id: return .foo
        case Self.bar.id: return .bar
        case Self.baz.id: return .baz
        default: fatalError()
    }
}

I haven't looked whether we can allow the caller to customize the names of enum StaticMemberSwitchable or var switchable, but if possible, it would be nice to help callers avoid naming collisions.