P2PK scripts are not PKHash destinations, they should have their own
type.
This also results in no longer showing a p2pkh address for p2pk outputs.
However for backwards compatibility, ListCoinst will still do this
conversion.
Make sure that nothing else can change WitnessUnknown's data members by
making them private. Also change the program to use a vector rather than
C-style array.
CTxDestination is really our internal representation of an address and
doesn't really have anything to do with standard script types, so move
them to their own file.