Add `into_tuple` so that you can `co_await` a `when_all` sender in a coroutine

<a href="https://github.com/ericniebler"><img src="https://avatars.githubusercontent.com/u/225757?v=4" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [ericniebler](https://github.com/ericniebler)**
_Sunday Feb 20, 2022 at 21:12 GMT_
_Originally opened as https://github.com/NVIDIA/stdexec/issues/465_

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Could be in the same paper as #464 
