Get mutalbe references to two locations in Collection by key.
```rust use mmut::MutPair; let mut v = vec![1, 2, 3, 4, 5];
let (a, b) = v.two_mut(1, 3).unwrap(); *a = 8; *b = 7;
assert_eq!(v, vec![1, 8, 3, 7, 5]); ```