propagate depth that was reached, not just whether there was a cycle

propagate depth that was reached, not just whether there was a cycle

I tried to propagate this information with the return value, but I
found a curiosity (actually, something that I'm not keen on in
general) -- in particular, the candidate cache urrently invokes
evaluation, which may detect a cycle, but the "depth" that results
from that are is easily propagated back out. This probably means that
the candidate caching mechanism *itself* is sort of problematic, but
I'm choosing to ignore that in favor of a more ambitious rewrite
later.