Blender Git Loki

Use a Scope concept for avoiding codegen for existing nodes.

Rather than a single socket value map, use a "scope" to keep track of which
nodes are already generated. This reintroduces the "block" idea from earlier
versions, but at the later codegen stage rather than the node graph.

Describing scope on the level of nodes becomes quite clumsy and ambiguous,
and moves nodes further away from the UI representation. The compiler can
handle this more elegantly without requiring persistent data.

Scopes can be nested. This is not used yet, but will allow scoped variables
later on, where the variables being used for a node path depend on its scope.