Float ord

[greyblake]

This PR addresses #28

It allows to derive Eq and Ord on float based types if they have finite validation:

use nutype::nutype;

#[nutype(validate(finite))]
#[derive(PartialEq, Eq, PartialOrd, Ord)]
pub struct Size(f64);


// Comparison
// 
let a: Size = Size::new(2.5).unwrap();
let b: Size = Size::new(3.3).unwrap();

assert_eq!(a.cmp(&b), Ordering::Less);
assert_eq!(b.cmp(&a), Ordering::Greater);


// Sorting
//
let initial_raw_sizes = vec![5.5, 2.0, 15.0, 44.5, 3.5];
let mut sizes: Vec<Size> = initial_raw_sizes
    .into_iter()
    .map(|s| Size::new(s).unwrap())
    .collect();
sizes.sort();
let sorted_raw_sizes: Vec<f64> = sizes.into_iter().map(Size::into_inner).collect();
assert_eq!(sorted_raw_sizes, vec![2.0, 3.5, 5.5, 15.0, 44.5],);