- 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.
- Equality, hashing etc all implemented automatically
- Default values are defined in just one place (the properties) instead of the constructor as well
- Added test to ensure that serialization works properly