profiles: Report the old and new version number in upgrades.

* guix/profiles.scm (manifest-lookup): New procedure.
  (manifest-installed?): Use it.
  (manifest-transaction-effects): Return a pair of entries for upgrades.
  (right-arrow): New procedure.
  (manifest-show-transaction)[upgrade-string, →]: New variables.
  Report upgrades using 'upgrade-string'.
* tests/profiles.scm ("manifest-show-transaction"): New test.
  ("manifest-transaction-effects"): Match UPGRADE against a pair.