Derive opcode_length instead of setting it explicitly

[zslayton]

          Can this be derived (i.e. `fn is_tagless() -> bool`) rather than storing the opcode length in a field? Tagless encodings _must_ have `value_body_length + length_length == total_length`, and anything else _must_ have an opcode, so `value_body_length + length_length < total_length` is always true for anything that is not a tagless encoded argument. Right?

Originally posted by @popematt in #801 (comment)

[zslayton]

Also look into addressing this comment:

Perhaps Header should actually be an enum so that it could have a NoHeader variant in addition to whatever other implementations you have going on?