[plt-scheme] xxx chooses MzScheme as preferred language

Most of the MzScheme benchmarks still appear to be `module'-less, and I
think most will benefit from a `module' wrapper.

For example, with `module' on my machine, fannkuch takes half as long,
and nseive takes 1.4 as long.

For the former, just start the code with "(module fannnuch mzscheme",
and a closing paren at the end, and change the "-r" in the run command
to "-u".

The latter is similar, except that the import of SRFI-13 has to be

   (require (only (lib "13.ss" "srfi") string-pad))

to avoid conflicting definitions.

Matthew