i.e. All values are anonymous strings until "queried"
- Unlike original PR, no change in existing test output for un-referenced properties
- unused variables would theoretically result in a tiny improvement in parsing time
- refactored how import reference works
- refactored to-css-visitor (this is side product, it was getting
complicated)
- fixes issues #1851, #1896, #1878, #2716, #1968, #2162 (same as #1896)
