CAY-1803  Define toString() in path expressions

and now actually rewriting the select ranslator to avoid toString that can theoretically return anything

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1457050 13f79535-47bb-0310-9956-ffa450edef68
warnings cleanup

git-svn-id: https://svn.apache.org/repos/asf/cayenne/main/trunk@1457051 13f79535-47bb-0310-9956-ffa450edef68