Racket v8.6 Release Thread

The release process for v8.6 will begin in about a week.  If you
have any new features that you want in and are relatively close to
to being done, now is a good time to do that.

Upcoming dates:
- 29th:  Branch day, merge window starts
- 15th: Merge window ends, testing starts
- 22nd: Testing ends

(NB: both this message and branch day are earlier than usual, to accommodate summer travel.)