Add insert and remove methods to vec::MutableVector<T> ?

[Dretch]

I think it would be useful to have functions to insert and remove items from arbitrary positions within a mutable vec.

In other words, to integrate this trait into vec::MutableVector;

pub trait VecInsertRemove<T> {

    /// Insert an element at position i within self, shifting all
    /// elements after position i one position to the right.
    fn insert(&mut self, i: uint, x:T);

    /// Remove the element at position i within self, shifting all
    /// elements after position i one position to the left.
    fn remove(&mut self, i: uint) -> T;

}

Is this desirable? I can send a pull request with these additions plus tests.

[catamorphism]

Fixed in 689f6ce -- thanks!