Allow goals to yield to other goals while working

Allow goals to yield to other goals while working

Previously we searched over streams of states, that provided solutions
to the searched sub-goals. This meant though, that if you attempted to
search in a goal which couldn't make progress like the "unproductive"
rule below, it would block forever with no way to yield back to the rest
of the search without producing a wrong result.

  unproductive(X) :- unproductive(X).

The new search works via iterators over Step instead of over State,
where a Step::Working represents a goal which was unable to produce a
result, and instead decided to yield, and a Step::Soln(State) is a
solution like before.

Currently the only goal which will introduce new Working states into the
stream is the delay constructor, before forcing, since recursive goals
are the only non-terminating goals that are in the core.