[TASK] PLONK check_membership_if_enabled gadget

[lazovicff]

check_membership_if_enabled is very similar to check_membership, except it takes in one more variable enabled:

fn check_public_set_membership_enabled<F, P>(
    composer: &mut StandardComposer<F, P>,
    set: &Vec<F>,
    member: Variable,
    enabled: Variable,
) -> Variable
where
    F: PrimeField,
    P: TEModelParameters<BaseField = F>,
{
    // Compute all differences between `member` and set elements
    let mut diffs = Vec::new();
    for x in set.iter() {
        let diff = composer.arithmetic_gate(|gate| {
	        gate.witness(member, member, None)
		        .add(F::zero(), F::one())
		        .pi(-*x)
        });
        diffs.push(diff);
    }
    
    // Accumulate the product of all differences
    let mut accumulator = composer.add_witness_to_circuit_description(F::one());
    for diff in diffs {
        accumulator =
	    composer.arithmetic_gate(|gate| gate.witness(accumulator, diff, None).mul(F::one()));
    }
    
    accumulator = composer.arithmetic_gate(|gate| gate.witness(accumulator, enabled, None).mul(F::one()));
    composer.is_zero_with_output(accumulator)
}