Add code to compute in-order mappings for tokens.

Add av1_indices_from_tree() function that computes a forward and inverse
 mapping of the tree leaf-node symbols to their in-order traversal.
This is necessary because many of the aom_tree binary trees have their
 leaf nodes out of order (e.g., an in-order traversal of a tree with n
 nodes does not start at symbol 0 and go to symbol n - 1), but the CDFs
 created by tree_to_cdf() are indexed in-order.

Change-Id: Icd0dbed4c171a67c9e84a634106c4fdb5b1b3488