Add module pyk.kore

[tothtamas28]

Closes #2626

This PR introduces the pyk.kore module. It contains:

• Class hierarchy for the Kore abstract syntax
• Parsing and unparsing in textual Kore format
• Parsing and unparsing in JSON format

Currently, only syntactic validation is performed on the AST (e.g. identifiers are lexically correct), semantic validation rules described in https://github.com/runtimeverification/haskell-backend/blob/master/docs/kore-syntax.md#validity are not implemented.

[radumereuta]

These parser files are very large. I don't feel confident reviewing them. At least not by myself.
Are they generated by any chance?

[tothtamas28]

Are they generated by any chance?

No. Since the Kore grammar is relatively simple, I wrote the parser manually.

[ehildenb]

Looks fine to me on first glance (no deep review of the Kore). I'm sure we'll find problems as we test, but no need to hang up the first version of it too long.

[ehildenb]

We likely will want to use some faster parser eventually, but we can cross that bridge when we come to it, and by then we'll have more test-cases we can use.