fuzzgen: Move Arbitrary structs into the fuzzers

[afonso360]

👋 Hey,

This PR is built on top of #5765, sorry about chaining these PR's so much, but doing it on top of main would cause conflicts with the two other PR's.

This PR does some cleanup around fuzzgen that @jameysharp sugested in #5764 (comment).

Namely:

• It moves the Arbitrary structs from fuzzgen into the individual fuzzers.
  • This also allows us to keep ALLOWED_LIBCALLS near the interpreter implementation, which need to be updated together.
• It centralizes the testcase printing infrastructure in a helper struct provided by fuzzgen.

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[jameysharp]

I think this turned out really well!

As a neat bonus, this also means that if we want statistics about how many times we rejected a fuzz input, like you were checking for a while, now we can reference static variables that are local to each fuzz-target from inside the implementation of Arbitrary.

After you rebase the other PRs out from under this one, go ahead and merge!

[afonso360]

Rebased this onto the latest main and fixed the issue above.

As a neat bonus, this also means that if we want statistics about how many times we rejected a fuzz input, like you were checking for a while, now we can reference static variables that are local to each fuzz-target from inside the implementation of Arbitrary.

Yeah, that's a great idea! I'll put a PR up for that!