Circuit type alias for Protocol

[robinhundt]

Doing the recent refactor #29 was painful because all the different circuit and iterator types needed to be changed. I think it is better to have these types be generic over the Plain and Gate type, but wen can increase the usability and maintainability by introducing type aliases of the form

type CircuitP<P> = Circuit<<P as  Protocol>>::Plain, <P as Protocol>::Gate<<P as  Protocol>>::Plain>>;

which allows us to write CircuitP<BooleanGMW> instead of the current Circuit<bool, BooleanGate>. This retains the flexibility, the underlying types are not changed, but increases usability and maintainability for those places where we can bound the circuit by a protocol (of which there are many).

[robinhundt]

Maybe a new like type PCircuit<P>, standing for ProtocolCircuit would be better. Or something else.