This also adds a new semantically different `address` method which
attempts to produce an address given the specified diversifier index,
and returns `None` if that index does not produce a valid diversifier.
It is then up to the caller to search the diversifier index space if
the diversifier index that they are attempting to use does not result in
a valid diversifier.
I misunderstood the meanings of Err(_) vs Ok(None), which indicates that
we need to refactor this API! In the meantime, this enables the example
to work with testnet Sapling viewing keys.
Kris Nuttycombe
Jack Grigg