Remove left_child and right_child from GTreeNode in favor of parent.

Remove left_child and right_child from GTreeNode in favor of parent.

For persistence to work, a node needs to know where its incoming pointers are.  If node->right/left can point to the next/previous node in the tree, then we might have an incoming pointer from our previous node, but don't have a pointer to it.  Finding the next/previous node takes up to log(n) time, which isn't cool to do every time we change a node.  So, if we want next/previous pointers, we need to always have the pointer, so that it can be recipricated, and thus would require two extra pointer spots beyond left/right to store that.

Because a node points to its children, they also need to be able to find their incoming pointer in their parent.  We add a pointer for this, which allows us to still navigate the tree, and thus we don't need to add next/previous pointers also.

Also remove test code that tests the right/left pointers as next/prev.