On cond-expand, once again

[Martin Grabmueller]

Hello again,

I have now reworked cond-expand wrt. the module system, but not
committed yet.  Modules can now provide features as a list of symbols
using the procedure:

  ;; Add one or more features to the `cond-expand' feature list of the
  ;; module `module'.
  ;;
  (define (cond-expand-provide module features)
          <snipped>)

It is used like this (taken from srfi/srfi-2.scm):

  (cond-expand-provide (current-module) '(srfi-2))

For ease of implementation, I have decided to just keep the mapping of
modules to features in a hash table.  `cond-expand' can then check
whether one of the modules in the use list of the current module
provides a given feature.  This works well for me, and I think is a
good solution.  Unless, of course, we wish to avoid module system
magic.

If nobody objects, I will simply commit it and you can have a look for
yourself.

Regards,
  'martin