Automatic transactions

If #38 is done, then it would be logical to allow contract authors to create saved transaction bodies, similarly to saved global contracts, that can be called in the non-Haskell environment of the transaction message, which disables the capability to write nonterminating code. These "canned" transactions can then serve as a safe interface to the contracts they expect to be called on, so long as their signatures are private types and can't be duplicated by another author.