Additional tests and fixes for assignment inlining (JENA-780)

- Don't inline into EXISTS/NOT EXISTS since those are n-ary operators
- Permit inlining within an EXISTS/NOT EXISTS only if the assignments
  are subject to all the normal restrictions
- Don't remove projection when inlining/eliminating through projections
  since doing so could cause previously hidden variables to become
  visible
- Various additional test cases for these
4 files changed