fixes issue where GUI would hang when trying to
navigate to display a choice tree (i.e. prompt)

this is a forward-looking change to support more
expressive control flow synchronization

this is an intermediate step in adding additional ways that a node
can be queued to be processed - in particular, handling node merge
logic via the scheduler

This clears up some of the ambiguity in the SyncPoint data which otherwise
had lots of implicit assumptions about the run-time contents of the NodeEntry
for each sync point

A SingleNodeEntry is functionally equivalent to a NodeEntry that's
annotated with the fact that it's single-sided

…r patched

this is essentially the same as Const, but useful for cases where
we want to establish OrdF or TestEquality

this now builds, and changes the control flow synchronization
to occur as a first-class operation that is explicitly scheduled

this provides some flexibility for handling additional cases
for mismatched control flow, but this is still work in progress

appropriately merged

this allows us to handle the "zero step" sync
case explicitly, where one of the sides of the
analysis takes no steps before re-synchronizing
control flow

… point