Add method to access P2PKH and P2SH hash arrays.

This change adds the Hash160 methods to AddressPubKeyHash and
AddressScriptHash so the hash may be accessed as an array, rather than
a byte slice with the ScriptAddress method of the Address interface.
In situations where arrays are more appropiate than slices (such as
for map keys), accessing the array directly this way can significantly
improve performance by reducing copying into local arrays.