Those oop closures are used only by defNewGeneration.cpp, group them there. This means genOopClosures can be removed.

Semantically, there are five kinds of closures used for oops in various places/situations.

- OldGenScanClosure: used on dirty cards in old-gen (old -> young)
- PromoteFailureClosure: used for promote-failure on eden/from spaces
- YoungGenScanClosure: used on to-space
- RootScanClosure: used on locations outside heap
- CLDScanClosure: used on locations in CLD

It's clearer to have concrete types and one can different assertions in each of them.