[plt-scheme] constants vs. literals in slideshow code

The "code.ss" library (in "texpict", also included in "slideshow")
uses the same color for constants and literals (a dark green by
default).  But SLaTeX uses different colors for these-- the MzScheme
manual, for example, uses red for constants, i.e. procedures
provided by the "mzscheme" module.

Any chance of adding "current-const-color" before my talk on Monday?  :)

--dougo at place.org