Dex: Internals continued

[bkase]

Dependencies tracked in other issues

• Make address a variable in zkapp circuits #136 for reusing a smart contract for two different accounts
• Generalize method arguments to allow auxiliary data #363 for passing in a verification key to a method; and maybe for passing an account update without introducing another special case for method arguments
• Set caller fields correctly #431 -- account update structure not working as expected
• [BUG] - Cannot call a @method that returns a value directly from a transaction. #429 -- return values to transaction block

This issue

• properly prove that we're executing a callback with certain arguments, in the zkApp A (see definitions below). this should be doable by directly using composability for A calling C, getting the party of C and wiring it up to be a child of B.

A - creates callback, passes it to B
B - gets callback, witnesses a superset of parties, does generic check on them
C - is called by the callback