I've left the iteration order of `MerklePath::inner_nodes()`
unspecified, but other methods of iteration and indexing are now
specified to be in order of deepest to shallowest.
Computation of the correct node indices to get is moved to
`NodeIndex::proof_indices()`, and getting a node's index based on its
parent is generalized into `SparseMerkleTree::get_hash()`.
