Return StackFuture instead of Box for async trait methods

[eholk]

This modifies dtolnay's async_trait macro to return StackFuture instead of Box to make it easier to simulate async traits with StackFuture.

Ideally, this should be re-exported from the stackfuture crate so you can use it like this:

#[stackfuture::async_trait(128)]
trait MyAsyncTrait {
    async fn foo(&self) -> i32;
}