Add unexport form for modules

[evhan]

An "unexport" form for modules would be useful. For example, this would only export the identifier "b":

(module m1 *
  (import chicken scheme)
  (define a 1)
  (define b 2)
  (unexport a))

This idea came out of #1376 and ​associated discussions on the mailing list. This would be a counterpart to "export", used to adjust the export list of a module from within its body.

megane has posted a patch on ​chicken-hackers, but it will need to be updated to apply to chicken-5.