[plt-scheme] 200.3

The exp-tagged code in CVS for MzScheme and MrEd is now version 200.3.

Between the initial 200.2 tagging and this 200.3, I exp-tagged many
minor improvements. Those changes include the "indentation suggests..."
reports for certain unmatched- and mismatched-parenthesis errors.

Changes specific to 200.3:

 * Added 'gif/mask and 'unknown/mask bitmap types, and added
   `get-loaded-mask' and `set-loaded-mask' methods to bitmap%. When
   loading a GIF file that contains a transparent pixel, and when the
   requested type is 'gif/mask or 'unknown/mask, then a mask bitmap is
   generated and installed as the "loaded mask".

   The mask bitmap has a white pixel for each transparent pixel in the
   main bitmap, and a black pixel everywhere else. It is not used
   automtaically when drawing the bitmap, but it can be extracted and
   used as the explicit mask argument to some drawing methods.

 * Added `exn:special-comment', which is to be used by custom input
   ports that contain non-character-based comments (e.g., the comment
   boxes soon to appear in DrScheme).

 * Extended `read-one-special' in readable-snip<%> in parallel
   to the exn:special-comment change for custom input ports.
 
 * Added 'packages-ok style flag to `get-file' and `get-directory'.
   Affects only Mac OS.

 * Lots of minor Mac OS improvements (X and Classic).

 * Inside MzScheme: fixed namespace initialization in
   scheme_basic_env() when it's called more than once. (This bug caused
   problems for MzCOM.)

Matthew