Callbacks: deferral support for single callbacks
Callbacks: deferral support for single callbacks

As in [Lwt.bind p f] when [p] is already fulfilled.

ocsigen/lwt,ocsigen/lwt,c-cube/lwt,rneswold/lwt,rneswold/lwt,c-cube/lwt,rneswold/lwt,ocsigen/lwt,c-cube/lwt