Add Mutex::into_inner()

``` rust
impl<T> Mutex<T> {
    pub fn into_inner(self) -> T {
         ...
    }
}
```

This is very useful for structures like this:

``` rust
struct Foo {
     data: Mutex<Bar>
}

impl Foo {
    fn modify(&self) {
        self.data.lock().unwrap().modify();
    }

    fn consume(self) {
        let bar = self.data.into_inner();    // oops! doesn't exist
        bar.consume();
    }
}
```
