Re-sampling functions for weighted sampling

Example

use wheel_resample::resample;

let mut rng = rand::thread_rng();
let weights = [0.1, 0.2, 0.3, 0.8];
let population = vec![1, 2, 3, 4];
let samples = resample(&mut rng, &weights, &population);

assert_eq!(samples.len(), population.len());

// Make sure all samples are in the population
assert!(samples.iter().all(|s| population.contains(s)));