fix: make sink::With send after inner sink returns ready

[quasi-coherent]

If I understand how Sink is supposed to work, a call to start_send has to immediately succeed a call to poll_ready that returns Poll::Ready(Ok(()). Any yield between poll_ready and start_send invalidates readiness of the sink because it could become unavailable during the intermediate polling. At least that's what seems "right" and how I read the documentation.

Currently in With, the future containing the next item is polled after the inner sink's initial Poll::Ready, which could mean that it becomes un-ready while polling for the item, then sent the value when it is unavailable.

The PR changes the helper method to call poll_ready on the inner sink, then immediately start_send the item. Unfortunately that means that the item has to be buffered in the intervening time, which leads to breaking changes in the Debug and Clone implementations.

The PR that introduces the current implementation seems to pretty plainly say that it's following the sink semantics, whereas before it was not. But before !1880 is more like what this PR is doing, so I could be way off. That would be fine too, since I'd appreciate having any of my misconceptions of Sink corrected.