Blender Git Loki

Nodes: support sharing node declarations between nodes

Previously, every node had its own declaration. This isn't ideal, because
it's often the case that all nodes of the same type have the same declaration.
That's the case for all nodes using declarations currently. It will not be true
for e.g. group nodes in the future.

Sharing node declarations between nodes makes it a bit more efficient.

Differential Revision: https://developer.blender.org/D12898