Add proc macro for converting async traits to return StackFutures

The idea here is to make a macro similar to the #[async_trait] one from the async trait crate. Using it would look something like this:

#[stackfuture::async_trait(4096)]
trait Foo {
    async fn bar(&self) -> i32;
}

The macro would then expand to something like (along with whatever lifetime annotations are needed:

trait Foo {
    fn bar(&self) -> StackFuture<i32, { 4096 }>`;
}

We'd do a similar thing for impls as well.

Start by reviewing the existing StackFuture API and the async_trait crate behavior referenced in the issue. Define how the proc macro should expand both trait declarations and impls, including required lifetime annotations; done means both forms produce working StackFuture-returning methods with the requested capacity.