`&mut [T]` should implement `Collection`

[japaric]

The following code should work, but it doesn't:

// Newtype wrapper to represent a row vector (or the row of a matrix)
struct Row<D> {
    data: D,
}

impl<D: Collection> Collection for Row<D> {
    fn len(&self) -> uint {
        self.data.len()
    }
}

fn main() {
    let mut v = vec![0u8, 1, 2];

    // Works because &[T] implements `Collection`
    println!("{}", Row { data: v.as_slice() }.len());
    // Works because &mut [T] gets reborrowed as &[T] (I think)
    println!("{}", v.as_mut_slice().len());
    // Doesn't work because &mut [T] doesn't implement the `Collection` trait
    println!("{}", Row { data: v.as_mut_slice() }.len());
}

Output

len.rs:20:20: 20:56 error: failed to find an implementation of trait core::collections::Collection for &mut [u8]
len.rs:20     println!("{}", Row { data: v.as_mut_slice() }.len());

I can send a PR.