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)

(https://github.com/clearmatics/zecale/pull/39#discussion_r494879213)