Trying to rework the TLV streams to have a more homogenous interface to work
with. This is by no means a complete implementation, just the groundwork that
is going to be used by the wire code generator to generate the specific
accessors, but it's enough so we can manipulate TLV streams in the onion and
later just switch to the generated ones.
The generated wrappers will ignore the raw fields and will only consider the
shortcut fields. This function takes the raw fields and serializes them
instead.