- Made all the mechanics of grammar parsing (GBNFGrammarParser, ParseState) internal. Just call `Grammar.Parse("whatever")`.
- Added a `GrammarRule` class which validates elements on construction (this allows constructing grammar without parsing GBNF).
- It should be impossible for a `GrammarRule` to represent an invalid rule.
- Removed some unnecessary `ToArray` calls
- Initial pass on LLamaStatelessExecutor, the context overflow management is broken but I think I found where it's ported from