Nodes: move some code from blenkernel directory to nodes

This also introduces the `blender::nodes` namespace. Eventually,
we want to move most/all of the node implementation files into
this namespace.

The reason for this file-move is that the code fits much better
into the `nodes` directory than in the `blenkernel` directory.

source/blender/nodes/function/node_function_util.hh

@@ -27,11 +27,11 @@
 #include "DNA_node_types.h"
 
 #include "BKE_node.h"
-#include "BKE_node_tree_multi_function.hh"
 
 #include "BLT_translation.h"
 
 #include "NOD_function.h"
+#include "NOD_node_tree_multi_function.hh"
 
 #include "node_util.h"