feat(runtime-core) Allow dynamic signature for polymorphic host functions

[Hywan]

Extracted from #1018.

⚠️ This PR targets #1217, not master! ⚠️

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This patch adds a new field in Func: signature. It contains the
signature of the host function.

For non-polymorphic host functions, the signature is computed from the
Args and Rets implementation parameters at compile-time.

For polymorphic host functions though, to be fully dynamic, the
signature given to new_polymorphic is used in Func as the correct
signature.

The improvement is the following:

- Func::<i32, i32, _>::new_polymorphic(…)
+ Func::new_polymorphic(…)

with this approach, we are more dynamic, and thus more easily embeddable in dynamic languages such as Python, Ruby, PHP and so on.

[losfair]

Can we remove the type parameters Args and Rets on Func, and instead place them on Func::new? Seems that with this PR we don't need the static type information any more?

[Hywan]

I'm afraid it will cause a regression. Let me think about it.

[Hywan]

Yeah, in our doc, we have code like let add_one: Func<u32, u32> = instance.func("add_one")?;. This code will be broken.

[losfair]

Yeah this is a breaking change.

Can we add something like ErasedFunc that provides a type-erased Func then (and maybe implement From<Func<...>> for ErasedFunc to keep the current API unchanged)? Having type parameters that do not always match the actual function type sounds confusing to me.

[Hywan]

It does also sound confusing to me.
The ErasedFunc idea is good. I'll update the patch.

[losfair]

Merging this first, will continue in #1217 .