Re-Implement resumable functions in wasmi

[Robbepop]

@tomaka provided us with a compelling argument for supporting resumable functions in wasmi again: #536

We removed the resumable functions feature in wasmi version 0.14 which also is the release in which we moved from the so-called legacy wasmi engine to the new reimplemented wasmi engine.
For simplicity and also because we were not aware of any important use cases back then we removed support for resumable functions. However, even though the new wasmi engine works quite differently and has a substantially different interface we are confident that resumable functions can be re-implemented for it.

This issue tracks progress and accumulates design ideas, issues and rationals.

edit: legacy wasmi API for resumable functions: https://docs.rs/wasmi/0.13.2/wasmi/struct.FuncInvocation.html

[tomaka]

Small note:

Wasmtime also doesn't provide a "resumable functions-like" API. In order to work around this in smoldot, I've implemented a "resumable functions-like" API on top of it. However, this comes at a pretty significant performance cost and is only possible because wasmtime allows you to bind asynchronous functions.
So this issue kind of relates in a way to a feature that lets you bind asynchronous functions.

However, to be clear, to me the superior API is the "resumable functions" one. Any other API can be implemented on top of it with (in theory) a negligible performance cost.

[Robbepop]

The design in #598 is as follows:

impl Func

enum ResumableCall

struct ResumableInvocation