Worth changing?

- Perhaps it would be better to make 2 commits (?): one for adding
  ‘shifted-generation’ and ‘switch-to-generation’ procedures to (guix
  profiles) and another is for adding the “--switch-generation” option
  itself.

- Also I made a couple of cosmetic changes in “guix/scripts/package.scm”:
  * ‘filter-map’ was replaced by 'for-each' because it was called only for
    side effects there;
  * ‘begin’ was removed from ‘cond’.
  I think these changes do not deserve a separate commit and may stay in
  this patch.  Is it OK?