Add a mechanism for clients to store metadata for no-recreate translation

Today, clients have to decode from cache instructions to deduce how to
restore state (combined with heap-stored metadata they may have kept). The
DR core, however, uses instrlist recreation metadata as part of its state
updates for mangling expansions, but does not make it easy for clients to
do the same. This issue covers trying to add a convenience mechanism to
pass data from the bb event w/ translating=true to the restore_state event.