`OldNewMap` used to have its own map implementation. Given that the file uses C++ now, it is easy to use a C++ map implementation instead. This simplifies the code a lot. Going forward it might make sense to remove the `OldNewMap` abstraction or to split it up in two (currently, `NewAddress.nr` has two different meanings in different contexts which is confusing). No functional changes are expected. Differential Revision: https://developer.blender.org/D16546