ConciseMidTag human-readable serialization

- Add ConciseMidTagBuilder for constructing valid ConciseMidTag tags.
- Serialize ConciseMidTag using string field names for human-readable
  formats, and integer keys otherwise.
- Rename "extension" field to "extensions" to be consistent with
  analogous fields in other structures and with the field's type.

Signed-off-by: setrofim <[email protected]>

Author: setrofim