This is a complete rewrite of the derived node tree data structure. It is a much thinner abstraction about `NodeTreeRef` than before. This gives the user of the derived node tree more control and allows for greater introspection capabilities (e.g. before muted nodes were completely abstracted away; this was convenient, but came with limitations). Another nice benefit of the new structure is that it is much cheaper to build, because it does not inline all nodes and sockets in nested node groups. Differential Revision: https://developer.blender.org/D10620