In this commit, we export the ReadElements and WriteElements functions.
We do this as exporting these functions makes it possible for outside
packages to define serializations which use the BOLT 1.0 wire format.
This commit adds the new set of single funder messages from the
specification. As a result, after this commit and a follow up, all of
our messages will directly line up with those that are detailed within
the specification.
The new set of funding messages are very similar to our prior ones,
aside from the main difference of the addition of several channel level
constraints that give nodes control over their exposure, throughput,
and other values.
