fast dhash

A fast rust implementation of the perceptual hash "dhash".

The main difference with other rust implementations, and the reason it is called "fast", is that it doesn't use grayscale and resize_exact image methods, therefore running about ~50% faster

basic usage

```rust use fast_dhash::Dhash;

use image; use std::path::Path;

fn main() { let path = Path::new("../image.jpg"); let image = image::open(path);

if let Ok(image) = image {
    let hash = Dhash::new(&image);
    println!("hash: {}", hash);
    // hash: d6a288ac6d5cce14
}

} ```