Re-land "add IntoFuture trait and support for await"

[tmandry]

#65244 was reverted in #67768 for performance reasons (see #67706). This issue tracks re-landing the support in a more performant way.

Tracking issue for IntoFuture trait: #67644

cc @seanmonstar @wesleywiser

[seanmonstar]

Sorry for my lack of rustc internal knowledge, is there any hint or guess at why it was slower?

[tmandry]

I don't think anyone knows for sure at this point. #67706 mentions normalize_ty_after_erasing_regions as the query that regressed.

cc @rust-lang/wg-traits

[tmandry]

In the @rust-lang/wg-async-await meeting we agreed that this was a nice to have, but would be good to get it over the finish line. Tagging this as OnDeck, because we'd like to look into it while people still remember the implementation details.

[jonas-schievink]

The same query regressed by 10% in #68524, also without being invoked more often.

[bstrie]

#68606 alleges to unblock this issue, would anyone like to benchmark an un-reversion to see whether or not that's true?