Add tracking issue for #!feature(async_closure)]

[ultrasaurus]

This RFC isn't fully implemented, but tracking bug is closed.
This PR adds a tracking bug I found for part of the RFC that is not implemented.
There may be other open tracking bugs. This PR is intended to be an improvement,
may not be a complete fix for issue: rust-lang/rust#66909

[nikomatsakis]

We don't always keep RFCs up to date with all bugs. I'm going to close this issue to avoid setting a precedent, but thanks for the thought.

[ultrasaurus]

FYI - There are still few reliable resources for people to learn about new features that are considered "stable." I would think Rust community might be more concerned with providing information than setting precedent.

If someone knows how to file a process improvement, that would be nice.

[Lokathor]

Yeah I've more than once had the problem where it's really hard to find information about the actual state of things, particularly with all the old versions of the rust docs and stuff clogging up the google results.

[nikomatsakis]

@ultrasaurus in retrospect, I agree. Sorry! I was going through a long triage list at the time and I think I misunderstood what was going on here.