Callbacks: track callback stack depth
Callbacks: track callback stack depth

Before this commit, Lwt only tracked whether it was already inside a
callback call or not, i.e. a single boolean value. Now, it tracks how
many callback calls it has nested.

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