Add height method body for Sparse Node

[bvrooman]

The PR #55 adds a height method to the common::Node trait. This method is required to return the height of the node in the tree. When retrieving the path from a root to a leaf, the instruction set is read from the leaf bits starting at the index corresponding to the root height. The root node reports its height to the iterator.

The Sparse::Node struct implements common::Node and must therefore implement the height method. Currently, the struct does not store its height or have any way of calculating it. Therefore, we must store the height of the node in the node buffer and expose it through a height method.