c-api: wasmtime_store_limiter

[LukasForst]

This was not yet discussed, but as of now, there's no way how to use StoreLimits from the C API. This PR adds wasmtime_store_limiter that allows setting supported limits for the Store.

Not sure who to assign for this PR, judging from the past PRs for c-api @alexcrichton maybe?

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[alexcrichton]

I think there's a case to be made for allowing precisely 0 of the above items, so could this instead perhaps use a negative value to indicate "use the default"?

[LukasForst]

I was thinking about this as well, but in that case we would need to use different types then usize and u32. What do you suggest using instead of these?

[alexcrichton]

For some of these values the range of 0..i32::MAX is sufficient (or even isize::MAX). Otherwise though you could also add a dedicated struct with getters/setters where not calling a setter is equivalent to "none" or something like that. I don't know the best way to handle optional parameters in C myself.

[LukasForst]

struct is not perfect, you'd end up with same problems, even if you use pointers on integers.. because then (afaik) you wouldn't be able to distinguish 0 and NULL.

Let's go with -1 as indication "to leave default" and i64 for all types. That way we keep the max ranges same and we will have a way to use 0 as a correct value.

[alexcrichton]

Sounds reasonable to me!