Update the macros to properly account for arrays

[FL03]

extern crate acme;

use acme::autodiff;

#[test]
fn test_array() {
    let x = [1.0, 2.0];
    let y = [2.0, 2.0];
    assert_eq!(autodiff!(x: x + y), 1f64);
    // panics here
    // assert_eq!(autodiff!(x: x + y), [1.0, 0.0]);

}