add ids to nodes

[kevinbarabash]

An id has the following behavior:

• cloning an AST does not update the id of nodes
• need nodes created because of changes to the AST must be giving an id that has never been used before

The purpose of this is feature is to provide to indicate nodes which move because of a transform but are essentially the same node.

An alternative is to always create new ids whenever cloning but to also add a prevId which points to the value of the id for that node in the previous step. One benefit of this approach is that a node maybe copied several times (distribution) or collapsed into a single node (factoring).