Support non power of two sizes

[recmo]

On 2019-09-04 @recmo wrote in b51698e “Use size instead of depth + support zero”:

Support non power of two sizes

                commitment: Commitment::from_size_hash(size, &Hash::default()).unwrap(),
                nodes: MmapVec::with_capacity(0),
                leaves,
            });
        }
        // TODO: Support non power of two sizes
        require!(size.is_power_of_two(), Error::NumLeavesNotPowerOfTwo);
        require!(size <= Index::max_size(), Error::TreeToLarge);

        // Allocate result
        let leaf_depth = Index::depth_for_size(size);

From crypto/merkle-tree/src/tree.rs:75