This commit refactors the remaining usage of WriteElements. By
replacing the interface types with concrete types for the params used in
the methods, most of the encoding of the messages now takes zero heap
allocations.
This commit changes the WriteElement and WriteElements methods to take a
write buffer instead of io.Writer. The corresponding Encode methods are
changed to use the write buffer.
In this commit, we create a new `ExtraOpaqueData` based on the field
with the same name that's present in all the announcement related
messages. In later commits, we'll embed this new type in each message,
so we'll have a generic way to add/parse TLV extensions from messages.