Co-locate parser generator logic

[Xanewok]

Part of #638, removes the codegen_grammar crate

Instead of having the definitions and the parser generation separate, we now co-locate this and provide a parser-specific model (for MVC rendering introduced in #962) for the code generation. This mostly moves code around and documents a bit more what we already have.

This is fairly non-controversial and I am for #638 to be incremental, so this is meant to progress the issue rather than fix it in one giant PR to keep up the velocity and land it as soon as possible, accounting for my upcoming move to Hardhat soon. If this generally makes sense, I'd love to have a rubber stamp for it; for now, I'll work on top of this to further migrate off DSLv1 definitions.

[changeset-bot]

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