create () returns an empty ivar, create_full, a full one.
fill t v fills t with value v if t was empty. If t was full, fill raises
an exception.
It is guaranteed that immediately after calling fill t,
is_some (Deferred.peek (read t)).
fill_if_empty t v fills t with v if t is currently empty. If t is full,
then fill_if_empty does nothing.
is_empty t returns true if t is empty
is_full t returns true if t is full
peek t returns Some v iff t is full with value v.
value_exn t returns v if t is full with value v, and raises otherwise.