CAY-1803  Define toString() in path expressions

preliminary unit tests

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1457048 13f79535-47bb-0310-9956-ffa450edef68
CAY-1803  Define toString() in path expressions

actually reimplementing toString

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1457049 13f79535-47bb-0310-9956-ffa450edef68