Add async recursion helpers

[gabotechs]

Adds a new recursion helper that resembles .transform_up() but with an async API:

    async fn transform_up_async<F, Fut>(self, f: F) -> Result<Transformed<Self>>
    where
        Self: Sized,
        F: Fn(Self) -> Fut + Send + Sync,
        Fut: Future<Output = Result<Transformed<Self>>> + Send;

To be used in future PRs in this same stack.

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[jayshrivastava]

Nice. This is much more clear. Just one comment to fix the comment to document the Jump behavior.