lwt.ml: move unify
lwt.ml: move unify

...to be closer to the sequential composition functions. They are the
only code in Lwt that uses the unify helper.

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