[ZIP 308] Update ZIP to specify migration to Orchard

[daira]

I propose that Sprout -> Orchard and Sapling -> Orchard migrations be done in separate transactions. This reduces information leakage to a party who is observing the block chain but not the source IPs of transactions. Ideally we would submit the transactions over Tor.

[Orchard is provisionally the new name for the protocol we were calling Pollard.]