refactor(compute): testing chained operation calls with output labels

[10d9e]

Changes include:

• refactored CircuitBuilder that includes design for operations accepting circuit based inputs
• refactored mux gate
• created a circuit macro helper that maps operators and if/then/else to appropriate builder ops... available in both execute and compile modes

#[test]
fn test_macro_if_else3() {
    #[circuit(execute)]
    fn mux_circuit(a: &T, b: &T) -> T {
        let first_two = a - b;
        if first_two {
            a * b
        } else {
            a + b
        }
    }

    let a = 4_u8;
    let b = 4_u8;

    let result = mux_circuit(&a, &b);
    assert_eq!(result, a + b);

    let a = 5_u8;
    let result = mux_circuit(&a, &b);
    assert_eq!(result, a * b);
}

#[test]
fn test_macro_arithmetic_compiler() {
    #[circuit(compile)]
    fn multi_arithmetic(a: &T, b: &T, c: &T, d: &T) -> (Circuit, Vec<bool>) {
        let res = a * b;
        let res = res + c;
        res - d
    }

    let a = 2_u8;
    let b = 5_u8;
    let c = 3_u8;
    let d = 4_u8;

    let (circuit, inputs) = multi_arithmetic(&a, &b, &c, &d);
    let result = get_executor().execute(&circuit, &inputs, &[]).unwrap();
    let result: GarbledUint<8> = GarbledUint::new(result);
    let result: u8 = result.into();
    assert_eq!(result, a * b + c - d);
}

closes #31
closes #3
closes #2