Add StreamingIteratorMut and DoubleEndedStreamingIteratorMut

These are extensions of their non-`Mut` counterparts, in short:

```rust
pub trait StreamingIteratorMut: StreamingIterator {
    fn get_mut(&mut self) -> Option<&mut Self::Item>;

    fn next_mut(&mut self) -> Option<&mut Self::Item>;

    fn fold_mut<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, &mut Self::Item) -> B;

    fn for_each_mut<F>(self, mut f: F)
    where
        Self: Sized,
        F: FnMut(&mut Self::Item);
}

pub trait DoubleEndedStreamingIteratorMut:
    DoubleEndedStreamingIterator + StreamingIteratorMut
{
    fn next_back_mut(&mut self) -> Option<&mut Self::Item>;

    fn rfold_mut<B, F>(mut self, init: B, mut f: F) -> B
    where
        Self: Sized,
        F: FnMut(B, &mut Self::Item) -> B;
}
```

Only `get_mut` is required, and the rest have default implementations.

There is also a new conversion from `Iterator`:

```rust
pub fn convert_mut<'a, I, T: ?Sized>(iterator: I) -> ConvertMut<'a, I::IntoIter, T>
where
    I: IntoIterator<Item = &'a mut T>;
```

Author: cuviper