Racket v8.18 release has started

The release process for v8.18 will begin in about *two* weeks (I'm 
out of the country for a bit).  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:
- 15th:  Branch day, merge window starts
- 23rd: Merge window ends, testing starts
- 30th: Testing ends