Switched memory to using shared nodes, and node_data to keep only naked node pointers, not shared nodes (to break the cycle, and we don't need weak pointers because their memory is guaranteed to exist, via 'memory')

Added (another) layer - now 'node_ref' is between node and node_data, and it decrees whether nodes are identical

Switched the 'pragma once' to only happen on MSVC, or gcc >= 3.4 (it was causing trouble on the sun compiler)