A crate to add two more sorting methods
---

Say you have a vector of tuples, which you would like to sort by one of the
elements (maybe you got it by using the enumerate or zip iterator methods,
then collected into a Vec). Now, if you write the following, you will get
lifetime errors:
```
let mut letters = vec![
    (1, "C".to_string()), 
    (2, "B".to_string()),
    (3, "B".to_string()),
    (4, "A".to_string())
];
letters.sort_by_key(|x| &x.1);
println!("Letters: {:?}", letters);
```
So you might try not borrowing, but then you realize String isn't Copy:
```
let mut letters = vec![
    (1, "C".to_string()), 
    (2, "B".to_string()),
    (3, "B".to_string()),
    (4, "A".to_string())
];
letters.sort_by_key(|x| x.1);
println!("Letters: {:?}", letters);
```
So this fails because you'd be moving out of the array! You don't want to 
have to expensively clone all of these strings just to compare them. That's
where this library comes in:
```
use sort_by_borrowed_key::SortByBorrowedKey;
let mut letters = vec![
    (1, "C".to_string()), 
    (2, "B".to_string()),
    (3, "B".to_string()),
    (4, "A".to_string())
];
letters.sort_by_borrowed_key(|x| &x.1);
let expected = vec![
    (4, "A".to_string()), 
    (2, "B".to_string()),
    (3, "B".to_string()),
    (1, "C".to_string())
];
assert_eq!(letters, expected);
```
That's it! The only other thing this crate adds is a unstable-sort version
of the same method, `sort_unstable_by_borrowed_key`, which is faster but may
change of the order of equal elements (see the docs for `[T]::sort_unstable`).

Many thanks to `danieldg` in `##rust` on freenode IRC for help understanding 
how to write this, including fantastic examples!