[plt-scheme] 200alpha20

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

The exp-tagged code in CVS for MzScheme and MrEd is now version
200alpha20.

Changes:

 * Added `quasisyntax', `unsyntax', `unsyntax-splicing' and reader
   macros 
       #`    #,     #,@
   with the obvious meanings analogous to `quasiquote', etc.

 * Added `make-file-or-directory-link' (Unix and Mac OS X only).

 * MrEd (and, therefore, DrScheme) runs in Mac OS X (Quartz) well
   enough to be useful. Build it via `configure', which expects to use
   the developer tools supplied by Apple. In particular `configure'
   generates makefiles for a Quartz build now, instead of an X11 build.
   (Use --enable-xonx to get an X11 MrEd.)

Matthew