retain_mut 0.1.1

Provide retain_mut method that has the same functionality as retain but gives mutable borrow to the predicate.
Documentation
# RetainMut

This crate provides trait `RetainMut` which
provides `retain_mut` method for `Vec` and `VecDeque`.

`retain_mut` is basically the same as `retain` except that
it gives mutable reference of items to the predicate function.

Since there is no reason `retain` couldn't have been designed this way,
this crate basically just copies the code from std with minor (1-line) change
to hand out mutable reference.
The code these impls are based on can be found in code comments of this crate.

This was probably a historical mistake in Rust library,
that `retain` should do this at the very beginning.
See [rust-lang/rust#25477](https://github.com/rust-lang/rust/issues/25477).

## Examples

### `Vec`

```rust
let mut vec = vec![1, 2, 3, 4];
vec.retain_mut(|x| { *x *= 3; *x % 2 == 0 });
assert_eq!(vec, [6, 12]);
```

### `VecDeque`

```rust
let mut deque = VecDeque::from(vec![1, 2, 3, 4]);
deque.retain_mut(|x| { *x *= 3; *x % 2 == 0 });
assert_eq!(deque, [6, 12]);
```