Previously, node types had a callback that creates internal links. Pretty
much all nodes used the same callback though. The exceptions are the
reroute node (which probably shouldn't be mutable anyway) and some
input/output nodes that are not mutable.
Removing the callback helps with D13246, because it makes it easier
to reason about which internal links are created and when they change.
In the future, the internal links should be part of the node declaration.
- Many cleanups of to use list base
- Some variable changes
These change is needed to migrate to the new socket builder API
Reviewed By: manzanilla
Differential Revision: https://developer.blender.org/D12366
