Add default values for symbolic API functions

[pgoodman]

Being able to provide "sane" default values for functions returning symbolic values (e.g. DeepState_Int) may be worth it, both for getting a basic test suite working without any input from a runtime engine / fuzzer, as well as potentially for "guiding" a runtime engine toward a particular value. In the latter case, the return value from a symbolic function could be a sort of OneOf({default_val, symbol}).