Add a way to resize StackFutures

首先阅读仓库中的 StackFuture 实现和现有测试，然后跟踪其存储大小和 ownership 的表示方式。评估提议的 resize API，包括成功路径和原始 future 的错误路径；完成的标准是：可以安全地尝试使用更小或更大的 StackFuture，而无需 boxing，除非 resizing 失败。

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.