[plt-scheme] Wrapping non-PLT code in modules

Any module can be a language; it's exports determine what initially in
the language. So, if you create a module and imports everything from
the r5rs language and re-exports it (you'll need to use renaming to
avoid conflicts with mzscheme) and then also re-exports include, you're
set.

You can even get more fancy and re-define the #%module-begin macro to
expand into correct includes and provides, so you can just write
something like this:

  (module my-r5rs-code "external-r5rs-lang.ss"
    ("r5rs-file-one.ss" names that-i-care about from-this-file)
    ("r5rs-file-two.ss" names that-i-care about from-this-other-file))

Robby