lookup_marshal_fun v This function takes a version v, and returns a
function that will downgrade (if necessary) the current version to v and
then write it to a bigsubstring. The contents of these buffers will be copied
immediatly, so it is safe to reuse the same bigstring for multiple
marshals.
lookup_unmarshal_fun v This function takes a version v, and returns a
function that unmarshals a message and upgrades it, returning zero or one
message as a result of the upgrade. The bigsubstring is only guaranteed
to contain valid data until the unmarshal function returns, after which it
may be overwritten immediatly.