Distinguish null address from address zero

[edfelten]

Currently ArbOS (and some aspects of its input protocol) treats a null address as the same as address zero. This is probably harmless, but it's better and safer to handle this according to spec. This would involve using option<address> in some places where the current code just uses an address.

(Null address is used in submitting txs to deploy contracts. Deploy txs use null as their destination address.)

[edfelten]

This should wait until after the module system upgrade is merged. The improved type-checking will make this easier to implement and test.