Rust: Provide a trait method to (optionally) control resource allocation

[cpetig]

This enables resource allocation from a user provided arena instead of the heap.

Arena implementation in test case generated by genAI.

[alexcrichton]

I like the idea, thanks! I've got some comments below, but let me know if you'd prefer to not handle anything in particular.

[alexcrichton]

@cpetig ok I tried my hand a this a bit and resulted in the last commit here. Everything looks good to me now but I'd want to run that by you to confirm.

I realized that ideally what I want is:

trait GuestThing {
    type Rep: ResourceRep = Box<Option<T>>;
    // ...
}

however defaults for associated bounds aren't stable in Rust. In lieu of that I did a, now unrelated, refactoring to the Option<T> "rep" which abstracts operations behind a trait. This is sort of like the _FooStorage<T> from prior but that didn't do what I want because it wasn't customizable. It's still not customizable due to the blanket impl, but I think it's a bit closer to being customizable (subjectively).

I also realized that the resource_into_raw_ method needed to be unsafe mostly because otherwise it would be possible to override just resource_into_raw_, nothing else, and you'll have written no unsafe code but that would be theoretical UB. This is best modeled as an unsafe trait but in lieu of that with associated types and such, I'll pass on that.

One possible future option is to basically require all Rust exported resources to manually say type Rep = Box<Option<Self>> as a form of boilerplate. That way it could be fully customizable at the cost of being a bit more verbose.