Add a way to resize StackFutures

まずリポジトリの StackFuture の実装と既存のテストを読み、その後、ストレージサイズと ownership がどのように表現されているかを追跡します。提案されている resize API を、成功パスと元の future のエラーパスを含めて評価します。完了条件は、resizing に失敗しない限り、boxing せずにより小さいまたはより大きい StackFuture を安全に試行できることです。

It'd be helpful to have a way to shrink (or less commonly, grow) a StackFuture. For example, sometimes you have an object that implements a trait and needs to forward calls to another object that implements the same trait. Right now that's impossible with StackFuture without boxing the child future because otherwise you'd have to have a future contain a future that's the same size as itself. That said, there will usually be some extra space so we might be able to dynamically shrink a StackFuture.

The signature would probably be something like:

fn resize<const NEW_SIZE: usize>(self) -> Result<StackFuture<T, NEW_SIZE>, Self>;

Then using this would look something like:

impl Foo for MyObject {
    fn bar(&self) -> StackFuture<(), 1000> {
        match self.sub_object.bar().resize::<{500}>() {
            Ok(f) => f.await,
            Err(original) => Box::pin(original).await,
        }
    }
}

The idea here is we'd try to fit the future into a smaller container, but if it doesn't fit then the resize returns the original future and we can either decide to pay the allocation cost (as we did in this example) or give up.