[plt-scheme] beware of new OS X bugs

OS X users of the CVS sources:

I've just tagged a substantial revision of plt/src/wxmac and related
files.

I fixed one memory-management bug, but that's all. Unfortunately, I
might have introduced *new* bugs, so please look out for anything
unusual in the next few days.

On the plus side, MrEd3m now works for OS X. 3m likely has bugs that
aren't in the non-3m version, but it looks good so far.

[MzScheme3m and MrEd3m are variants of MzScheme and MrEd that use a
 compacting collector instead of the usual "conservative" collector. As
 a result, space usage can be considerably lower, especially for
 long-running applications. 3m now builds for Unix variants and OS X.
 Windows support will be available in the near future. 3m is not
 currently suitable for applications that require C-implemented
 extensions.]

Matthew