Currently, we create a bunch of refs associated with our userdata (descriptors, timers, ZeroMQ watchers). Now that lua_newuserdatauv() exists, we could instead create userdata that way and keep the relevant Lua values alongside the C structures. struct watcher in lua_zmq.c in fact only exists to hold three lua values.

I imagine each type could have an enum of uservalue indices e.g.

enum watcher_uservalue {
  WATCHER_USERVALUE_SOCKET,
  WATCHER_USERVALUE_IN,
  WATCHER_USERVALUE_OUT,
  NUM_WATCHER_USERVALUES
};