Support native Rust applets

[ia0]

When the applet is written in Rust and it is not a security problem for the applet to be compiled as a native static library and linked with the platform (for example if the applet doesn't use unsafe and is cooperative), then users should be able to do so for performance, footprint, or other reasons.

A possible idea to do so would be to require the applet to be async. When a host function is called, the following would happen:

• The host function would store some information for the scheduler and yield.
• The scheduler would execute the function, store the result for the applet, and resume the applet.
• The host function would then see the result and return it.

Scheduling between applets and threads can only be done when applets call a host function. But given that this feature should be used when the applet is somehow trusted, it seems reasonable that it is also well-behaving and yielding regularly enough.

[ia0]

A simpler approach for a single applet would be to execute host functions without unwinding the stack. This means that the stack will grow proportionally to the WebAssembly threads stack (which shouldn't grow more than 5 threads for reasonable applets).