impl Clone for Cow

Now we can use `#[deriving(Clone)]` on structs that contain `Cow`.

r? @aturon or anyone else
> require T: Clone instead of B: BorrowFrom<T>

Note that the `Cow` *struct* enforces the `B: BorrowFrom<T>` bound, so it can't be removed. I wrote the `impl` in this way to reduce the numbers of bounds on the input parameters.