Clear division of roles of joinsplit_gadget and circuit_wrapper

[dtebbs]

It may be valuable to move the allocation of circuit inputs and the specification of the number of public inputs out of the gadget into the circuit_wrapper class. This is in order to:

• be consistent with other gadgets
• maintain better separation of concerns
• support reuse of joinsplit_gadget in other circuits (e.g. in the case of a single public input which is the hash of the joinsplit inputs)

(clearmatics/zecale#39 (comment))