752adff9d1 Add method calc_height (Tobin C. Harding)
46f5588646 Add unit test for calc_tree_width (Tobin C. Harding)
Pull request description:
Add a private `PartialMerkleTree::calc_tree_width` function and a unit test to test it.
ACKs for top commit:
apoelstra:
ACK 752adff9d1
clarkmoody:
ACK 752adff9d1
Tree-SHA512: 9c4ad9f6ff47d8faad1c7c1e977427f1528af2712ceffd05357d0c9117b5fdb7b2783afc00a75cb19b853bfbd7b3895baa3a3563bdc496593cc9b06ce80dbbf8