flatbuffers: [rust] Generated enums can cause undefined behavior

Just remembered that a discriminant absent from an C-like enum is considered undefined behaviour in rust. This means that all currently generated enums could potentially cause undefined behavior if someone adds a new enum field and sends that buffer to someone using a older version of the schema.

I think the ideal solution would be to use a standard enums and add the appropriate TryFrom implementation (e.g. TryFrom<u8>). Maybe we should returned the unmatched integer as the error type.