[WIP] Assign NodeIds outside of constructors

[powerboat9]

This patch moves node id storage/handling into a subclass, HasNodeId, and requires that nodes be explicitly assigned a node id before they can have their node id fetched.

Pros:

• Removes the need for a distinction between copying with/out preserving node id
• I'm pretty sure this is closer to what rustc does

Cons:

• Node ids have to be explicitly assigned -- NodeIdVisitor is created to handle this
• Nodes have to be copied before being assigned a node id, if the copy is to have a different node id -- a method for reassigning node ids could be added, but this seems like best practice anyways
• HasNodeId needs to be explicitly initialized in copy constructors for derived classes. Otherwise the result of a copy has no assigned node id, instead of copying the original node's node id

Despite the cons, this is probably the best method available for solving the how-do-we-copy-nodes-with-or-without-node-id problem, as the alternative seems to be playing wack-a-mole with copy-vs-reconstruct in the derive handling code.