[racket] MysterX poll

Thanks for your responses to the MysterX poll. Based on those
responses, here's the plan:

 * Mark the ActiveX parts of ActiveX as to be removed after v5.2.1
   (i.e. after the next release). No one claims to be using the part
   of MysterX to be removed.

 * Reimplement the COM parts (core and events) as `ffi/com'. The
  interface of `ffi/com' will not match `mysterx' exactly; a new
  implementation of `mysterx' will wrap `ffi/com' for non-ActiveX
  backward compatibility, but porting from `ffi/com' to `mysterx' will
  be encouraged.

I already have a first cut at the new `ffi/com', which is based on help
and advice from Ben Goetter. When Ben and I are ready, we'll merge to
the git repo --- after the branch for v5.2.1, but otherwise pretty
soon, I hope.