handle different lengths error case in point-mutations

[ekse]

The current tests for the point-mutations exercise assumes that both strings are of the same length. hamming_distance() should return an error when strings of different lengths are passed. I think the idiomatic way to this in Rust would be to change the function signature to return a Result

pub fn hamming_distance(a : &str, b: &str) -> Result<u32, &'static str> 

For expected tests cases a call to unwrap() needs to be added, for error cases we can compare with the error.

#[test]
fn test_no_difference_between_empty_strands() {
    assert_eq!(dna::hamming_distance("", "").unwrap(), 0);
}

#[test]
fn test_second_string_is_longer() {
    assert_eq!(dna::hamming_distance("A", "AA"), Result::Err("inputs of different length"));
}

I will send a pull request for this issue.