Adjacently and internally tagged enums for non-self-describing formats

[da-x]

It seems that supporting adjacently and internally tagged enums can only work for self-describing formats (see bincode-org/bincode#272). For some non-self-describing formats, it would be nice to support adjacently tagged structs by treating the tag and content values as a tuple instead of additionally serializing the tag string and the content string (i.e. elision of the tag and content strings from serialization and deserialization).

[Kobzol]

It seems that this is also a problem for msgpack-rust (3Hren/msgpack-rust#250), which serializes adjacently tagged enums as tuples.

[dtolnay]

I opened #2496 to make progress on one piece of this.