Update the Guide's performance section with current information for
Racket CS, and also document the Racket CS compilation mode and
inspection environment variables. Make a couple of environment
variables work more consistently: PLTDISABLEGC for CS and PLT_ZO_PATH
for BC.
This commit merges changes that were developed in the "racket7" repo.
See that repo (which is no longer modified) for a more fine-grained
change history.
The commit includes experimental support for running Racket on Chez
Scheme, but that "CS" variant is not built by default.
