[racket-dev] Why did DrDr get much slower?

The DrDr running time tripled recently. This doesn't seem to be a
result of any one thing getting slower, but instead of all the files
taking longer.  To pick two random examples,
`racket/lib/collects/racket/match/parse-quasi.rkt` went from ~1 second
to ~4 seconds, and `pkgs/realm/chapter2/source.rkt` went from ~1.5
seconds to ~3 seconds. You can see the charts here:

    http://drdr.racket-lang.org/27103/racket/lib/collects/racket/match/parse-quasi.rkt
    http://drdr.racket-lang.org/27103/pkgs/realm/chapter2/source.rkt

The change where this happened was Matthew's addition of
`filesystem-change-evt`, which was intended to make things faster.

This slowdown seems to be real on my machine as well --
realm/chapter2/source.rkt runs more than twice as slow in rev bd09a60e
as it does in v5.3.5. I'm testing the revision right before
`filesystem-change-evt` now.

Sam