Reconcile streaming approach with crawling grammar depth approach

[eionblanc]

Currently, streaming and crawling grammar depth are incompatible (hence the streaming branch, which contains all grammar depth features before the crawling approach was implemented). To reconcile these, the following needs to be implemented:
(i.a) Have is_admissible return a certifying model for string admissibility. This should be easy to implement: the structure is already recursive, so this is just a matter of back-propagating the model path of bool choices.
(i.b) In ConstraintGrammar, implement derivation of bool valuations corresponding to a rule path.
(ii) When num_solutions is more than 1, pass previous solutions in additional_constraints option as actual admissible strings. These will then be considered by the deeper grammars (in terms of depth unpacking) using components (i.a) and (i.b) to apply the correct bool assertions for the SMT file (necessary since bool paths are in general unrelated between different grammar depths).