Implement a gradient macro

[FL03]

Considering certain limitations of Rust when using procedural macros, an attribute macro will be better suited as they are permitted the full scope of whatever logic is encompassed by the function definition.

Usage

use acme::grad;

#[grad]
fn add(x: f64, y: f64) -> f64 {
    x + y
}

fn main() {
    println!("{:?}", add_prime(2_f64, 0));
}