Further work on constant folding (JENA-630)

Fixes an issue with not folding constants inside operators nested within an EXISTS/NOT EXISTS
Expands the test cases for this new feature
Adds it to the standard optimizer - currently off by default until the feature is ready

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1567220 13f79535-47bb-0310-9956-ffa450edef68