Make GuidedSearchIterator work with multiple nonterminals

[eErr0Re]

• NewProgramsIterator now checks if the subexpressions have the correct type.
• GuidedSearchIterator only returns programs that can be derived from the starting symbol.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.04%. Comparing base (0cb2514) to head (7e60b24). Report is 1 commits behind head on probe.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## probe #105 +/- ## ========================================== + Coverage 72.85% 73.04% +0.19% ========================================== Files 29 29 Lines 991 998 +7 ========================================== + Hits 722 729 +7 Misses 269 269 ```

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