[plt-scheme] 200alpha21

[Matthew Flatt <mflatt@cs.utah.edu>]

The exp-tagged code in CVS for MzScheme and MrEd is now version
200alpha21.

This version provides Mac OS improvements. It doesn't look much better
than 200alpha20 --- there are still many significant "surface" problems
--- but it shouldn't crash anymore in OS X. For Mac OS Classic, MrEd
builds with MPW and mostly works, but stand-alone MzScheme is no longer
supported for Classic.

Change:

 * `send-event' (for sending AppleEvents) is now in MrEd instead of
    MzScheme.

    The new "sendevent.ss" MzLib library exports a `send-event'
    procedure for programs that need to sometimes run in MzScheme.
    Currently, MzLib's `send-event' works only when run within MrEd. In
    a future version, `send-event' may work in OS X by calling out to
    /usr/bin/osascript.

Matthew