In order to prep for allowing TLV extensions for the `ReplyChannelRange`
and `QueryChannelRange` messages, we'll need to remove the struct
embedding as is. If we don't remove this, then we'll attempt to decode
TLV extensions from both the embedded and outer struct.
All relevant call sites have been updated to reflect this minor change.
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.
