Update testing_boiler macro to not need a nightly compiler

[FreezyLemon]

Addresses part of #233.

This makes the macro a bit more complicated but removes the need for the fn_traits feature.

Instead of

testing_boiler!((f64, f64), Beta);

#[test]
fn some_test() {
    let arg = (1.0, 2.0);
    try_create(arg);
}

it's now

testing_boiler!(a: f64, b: f64; Beta);

#[test]
fn some_test() {
    let arg_a = 1.0;
    let arg_b = 2.0;
    try_create(arg_a, arg_b);
}

[YeungOnion]

haha, same approach I took, playground
Thanks for this, that was quick.