Prove correctness in the presence of back-pressure

Feed forward designs without backpressure can use our LEC flow because the pipeline stages conceptually collapse for a single example (stateful elements are trivially passing through on a given clock).

When we have backpressure, we'll need supplementary proof that show we don't drop state on backpressure, and that state eventually makes it to the end of the pipeline.

This may be part of the broader notion of "sequential" LEC, I'm not sure, but certainly if we add backpressuring we'll want to prove those two properties about the pipeline we generate.

cc @meheff