Introduce another form of debugging dump for the evaluator, rendering the
complete dependency graph using GraphViz, including all dependencies and
values cached within the evaluator.
Perform online tracking of the (direct) dependencies of each request made
to the evaluator, and introduce debugging code to print these dependencies
as a tree (from a particular request).
Turn the `activeRequests` stack into a `SetVector` and use it to detect
cycles. This approach works for uncached requests, and lets us simplify
the external-caching protocol quite a bit because we no longer record
“in-flight” states. Simplify the cache as well.
Thanks to Graydon for the suggestion.
Meant as a replacement for the barely-started iterative type checker,
introduce a simpler "evaluator" that can evaluate individual requests
(essentially, function objects with some additional API), caching
results as appropriate and detecting cycles.
