[WIP] Fix infinite loop when computing parse tree for recursive nullable grammars

[hudson-ai]

Computing the parse tree (needed for getting captures) previously caused an infinite loop if the grammar has loops of nullable rules, where a nullable rule is any rule where every symbol on the RHS is nullable.

E.g., the grammar

  A -> A C
  A -> B
  A ->
  B -> A
  C -> 'x'

has nullable rules

  A -> B
  A ->
  B -> A

and there is a loop in these rules formed by A -> B and B -> A.

For a deeper discussion on this, see section "One Last Trap" of https://loup-vaillant.fr/tutorials/earley-parsing/parser

This PR solves the issue by keeping a set of visited nodes when computing the parse tree (or getting captures) and skipping any node that has already been seen. AFAIK, the only way this can happen is when we have these nullable loops, but admittedly I still have a lot to learn about the parser (so more eyes on this would be really helpful!)

This closes #863

[codecov-commenter]

Codecov Report

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

Project coverage is 60.33%. Comparing base (15ff21d) to head (867adbe). Report is 1 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #874 +/- ## ========================================== + Coverage 57.02% 60.33% +3.30% ========================================== Files 63 63 Lines 4575 4583 +8 ========================================== + Hits 2609 2765 +156 + Misses 1966 1818 -148 ```

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[hudson-ai]

Currently, my fix is to avoid infinitely recursing over children, but I suspect that it might be nicer to prevent accepting parses that have infinitely looping children to begin with. That being said, the behavior seems correct (in that captures are correctly being... captured).

@slundberg I would really appreciate some input from you on this as well as what the correct semantics of partial captures should be (I tried to preserve existing behavior, but I'm not entirely clear on whether that behavior was the intended one).