Check that, after a program has executed, all constrained choices were visited

[marcoct]

It is illegal to constrain the value of a random choice without also guaranteeing that that random choice exists. This requirement allows us to check that a set of constraints was valid, by checking that every constraint was visited during the execution. This is a critical error-reporting check, because otherwise common naming issues will go un-noticed.

[marcoct]

For example, in the initial tutorial notebook, the names "foo-$n" and "foo_$n" were mixed up.

[marcoct]

Done.

[marcoct]

I am removing this feature temporarily, because it was confusing during my re-write of the probabilistic program tracing with subtraces (https://github.com/probcomp/Gen.jl/issues/58). There is some interaction with the design of subtraces. For example:

• It seems reasonable to only check that all constrained (or proposed or intervened) top-level addresses were visited, and rely on sub-generators to check that their constrained sub-addresses were visited.

• However, if we remove constraints (or proposals or interventions) to a sub-trace using delete!, such that there are no more constraints on a given sub-trace, we need some of way of knowing that we no longer have to check that the top-level address of the sub-trace was visited.

• Perhaps we should record the full list of directives (constraints, proposals, or interventions) applied to the trace, keyed by the top-level address of each subtrace. Then, in every call to delete!, we can remove a record, and check if there are no more directives for that sub-trace address.