2014-04-19-10.28.06: src/lib/src/issue-unique-id.api: For improved typesafety, introduced Id as an opaque alias for Int, changed issue_unique_id to return Id, added an id_to_int: Id -> Int; (but deliberately no int_to_id: Int -> Id; !) and an id_zero: Id guaranteed never to be returned by issue_unique_id().  For improved convenience, duplicated all this stuff in pervasive.pkg.