lwt.ml: move callback-related functions (4)
lwt.ml: move callback-related functions (4)

...to group them.

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