Remove mutative dom indexing

  Instead of indexing the dom on diff using a mutuative operation
  on the nodes, we count the descendants of each node  and write
  this as a read-only variable on the node. This allows us to
  cheaply compute the tree-order index during the walk and has the
  added benefit of allowing the VNodes to be reused anywhere without
  having to clone the structure. We could now freeze the nodes and
  they would behave as immutable data structures.