recent DrScheme changes

[Robert Bruce Findler <robby@cs.rice.edu>]

  - added module browser; available from Scheme menu.
    to see drscheme's module structure, start from
    plt/collects/drscheme/private/start.ss (not drscheme.ss,
    that just has some bootstrap code and a dynamic-require
    of start.ss)

  - Milestone: DrScheme now has a shortcut for every
    alphabetic key (and even one for semi-colon!)

  - added overbar-notation style printing to drscheme so all
    rationals can now be printed in decimal notation.

Robby