Nodes: move NodeTreeRef functionality into node runtime data

The purpose of `NodeTreeRef` was to speed up various queries on a read-only
`bNodeTree`. Not that we have runtime data in nodes and sockets, we can also
store the result of some queries there. This has some benefits:
* No need for a read-only separate node tree data structure which increased
  complexity.
* Makes it easier to reuse cached queries in more parts of Blender that can
  benefit from it.

A downside is that we loose some type safety that we got by having different
types for input and output sockets, as well as internal and non-internal links.

This patch also refactors `DerivedNodeTree` so that it does not use
`NodeTreeRef` anymore, but uses `bNodeTree` directly instead.

To provide a convenient API (that is also close to what `NodeTreeRef` has), a
new approach is implemented: `bNodeTree`, `bNode`, `bNodeSocket` and `bNodeLink`
now have C++ methods declared in `DNA_node_types.h` which are implemented in
`BKE_node_runtime.hh`. To make this work, `makesdna` now skips c++ sections when
parsing dna header files.

No user visible changes are expected.

Differential Revision: https://developer.blender.org/D15491

source/blender/nodes/function/node_function_util.hh

@@ -23,4 +23,6 @@
 
 #include "FN_multi_function_builder.hh"
 
+#include "RNA_access.h"
+
 void fn_node_type_base(struct bNodeType *ntype, int type, const char *name, short nclass);