Add proc macro for converting async traits to return StackFutures

[eholk]

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.

[eholk]

I forked async_trait and make a prototype of this here: eholk/stackfuture-async-trait#1