Optional fixed fields don't compile

[rooooooooob]

bar = {
  ? 5: "five",
}

Will refer to field self.key_5 which doesn't exist. This should either be supported or it should panic during generation telling the user that it's not supported.

If we support it we could either(for --preserve-encodings=false):

1. Ignore it for serialization and parse it (but ignore) for deserialization.
2. Have a x_present: bool field.

For --preserve-encodings=true we could stick that x_present in the *Encodings struct like we do for .default support.