* 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.