`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
57613630c7