chore: remove defunct projects (afaik)

[nikomatsakis]

I believe these teams are defunct...

• Dyn upcasting (stable!)
• GATs (stable!)
• Keyword generics? cc @oli-obk @yoshuawuyts
• FFI unwind (stable?)? cc @rust-lang/wg-ffi-unwind

[BatmanAoD]

WG-FFI-unwind did leave some hoped-for developments unrealized, but I think it's fair to say that its initial purpose has been served and that a new WG can be formed if someone has the bandwidth and motivation to pursue the remaining work.

Specifically, there's a half-baked concept of "discardable stack frames" and how Rust can (or if it even should) interact with longjmp and its ilk. Since there's been so little activity in the WG repo and zulip channel over the last few years, I am assuming that having this formally specified and guaranteed is not a high priority for anyone.

[jieyouxu]

Changes look fine, waiting a bit for any responses.

[jackh726]

Hmm, Niko, I think you might have the base as an old commit :) e.g. #2214 landed months ago

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[jieyouxu]

(Pushed a rebase to resolve the trivial conflicts.)