[NFC] Make fuzzing params mutable

[kripken]

This only makes them mutable, but does not actually mutate them in any
way. The new file parameters.cpp is the same as the old parameters.h,
with the exact same values, but without constexpr etc. on them.

The goal in a later PR is to adjust these to get even more variety in
codegen. E.g. in a huge module we might want many heap type
declarations.

[kripken]

this avoids a compiler warning on "comparing signed and unsigned"

[tlively]

I would strongly prefer not using mutable global state like this. If we want the parameters to be mutable, can we pass them as a configuration object to TranslateToFuzzReader?

[kripken]

Sure, I can do that. But note that that object wouldn't be constant, or at least we'd want to make copies of it with changes (e.g. so that not all functions have the same depth and other params). Does that change your mind?

[tlively]

Ok, then can we associate different copies of the configuration with the different functions?

[kripken]

Yes. But then it seems odd to pass the configuration in, so I am thinking we'd define a global config in the generator, and create a new one per function and set it active while generating it?

[tlively]

Why does it seem odd to pass the configuration down to where it will be used? Would that just be too invasive?

[kripken]

If you mean passing constantly in every make*() call, then yes, that would be quite invasive. We'd have dozens (hundreds?) of such places. It seems natural to keep it global, the same way we keep the FunctionContext.

If you mean passing it in from the outside to the fuzzer (that is what I meant before), then it seems odd for the outside to request params but the fuzzer modify them (and the goal is for it to modify them in a dynamic manner).

[tlively]

Got it, I wasn't sure what you had in mind with "it seems odd to pass the configuration in."

...so I am thinking we'd define a global config in the generator, and create a new one per function and set it active while generating it?

This sounds good to me.

[kripken]

Ok, rewritten as a context with RAII functionality. The RAII part is not yet used as the global context is never replaced, to keep this initial PR NFC.