Since internal links are only runtime data, we have the flexibility to
allocating every link individually. Instead we can store links directly
in the node runtime vector. This allows avoiding many small allocations
when copying and changing node trees.
In the future we could use a smaller type like a pair of sockets
instead of `bNodeLink` to save memory.
Differential Revision: https://developer.blender.org/D16960
05ddc7daa2