Previously, this function would expect a callback function as parameter.
This behavior is now in Map.lookup_or_add_cb. The new version just
takes the key and value directly.
This adds two data structures that wrap a node tree. However, they work
on different abstraction levels.
`NodeTreeRef` is an immutable structure that makes working with a node
tree in C++ much more efficient and convenient. It supports various
queries efficiently, that are not easily possible using just `bNodeTree`.
`DerivedNodeTree` builds on top of `NodeTreeRef`. It contains a flattened
view on the node tree, i.e. with node groups being inlined. Every inlined
node still knows its "call stack". It supports pretty much the same queries
as `NodeTreeRef`.
Both data structures come with a dot graph exporter for debugging purposes.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D7628
