Add stream selection early exit

[414owen]

select_early_exit creates a stream that produces elements while either substream contains elements, and terminates when either substream runs out of elements.

Previous work

#1881
#1422

Benchmarks:

Old	New
1,065,438ns/iter	1,159,341ns/iter

Performance hit: +8.8%.

The select_streams benchmark is quite contrived, using long streams nested deeply under multiple Selects to produce statistically significant results. As such, I'm happy with the +8.8% performance hit, and would prefer it over a separate version of SelectWithStrategy just for early exit.

Side note, if you leave out the #[inline] over the is_finished function, you get 1,830,713ns/iter, or a +71.8% performance hit.

I've tried to specialize SelectWithStrategy over an exit strategy trait in this branch, but surprisingly, got a small performance hit.

I can't say for sure what's going on, but my best guess is that the performance difference between this branch and the one linked above is due to a layout change, rather than how the logic is encoded.

It would be possible to use something like stabilizer to know for sure, but I doubt we actually want to go that deep on this.

[414owen]

Clippy is failing (on master too!). Fixed here: #2598

[414owen]

I've decided that I need more granularity than when both finished/when either finished.

The new API is trait-based.
Why is it trait-based? Because I tried implementing it with a function, and this got the best performance by far.
The function-based api is here: 414owen@eba9a37
Presumably the trait instance functions were inlined more aggressively.

With this change, we actually improve performance, beating master by a whopping 3.4%.

Old	New
863,006 ns/iter	832,950 ns/iter

I don't really know why that is, but see my comment on layout changes in the PR description.