It should be an optional record instead of an fn option. In addition,
its tlv type is bumped to be 14 as this record is also included in the
`DynCommit`. If we use tlv type 0, it will create a conflict in the msg
`DynCommit`, which is fixed in the following commit.
Previously we encode all the fields plus extra data to the field
`ExtraData`, this is now fixed by encoding only unknown data to extra
data. For known records, they are already encoded into the message
fields.
In this commit, we add a new `TestMessage` interface for use in property
tests. With this, we'll be able to generate a random instance of a given
message, using the rapid byte stream. This can also eventually be useful
for fuzzing.
