Major factorization: implement all in BlitzViewImpl with class-dep aggInternal

That means everything was moved into the common code, where each child
classes have to implement now only two operations:
  - >> to add a new Folding operating to the stack
  - aggInternal to implement the folding computation
and that's all.
Minor: remove old imports
Minor: add missing override in VV
Minor: use optimisation for rcCombine
Add new class BlitzViewFlattenVs
Change addFlatten to use BlitzViewFlattenVs through BlitzViewImpl
Fix BlitzViewFlattenVs reducer ordering (again)