Transaction scripting

If #39 is done, then it is logical to extend the ability to call "canned" transaction bodies in the transaction message to a limited DSL scripting language that would allow deconstructing contract return values, assigning names to their values and to escrow IDs, and calling multiple canned transactions on the results, without enabling the general-purpose features of Haskell that allow nontermination. Transaction scripts would therefore be guaranteed safe and would not require countersigning by various parties simply to ensure execution safety.