Add Iterator util methods/traits

This pull request adds following methods and traits.
```rust
pub trait IteratorUtil {
(snip)
    fn filter_map<'r,  B>(self, f: &'r fn(A) -> Option<B>) -> FilterMapIterator<'r, A, B, Self>;
(snip)
    fn to_vec(self) -> ~[A];
    fn nth(&mut self, n: uint) -> A;
    fn first(&mut self) -> A;
    fn last(&mut self) -> A;
    fn fold<B>(&mut self, start: B, f: &fn(B, A) -> B) -> B;
    fn count(&mut self) -> uint;
    fn all(&mut self, f: &fn(&A) -> bool) -> bool;
    fn any(&mut self, f: &fn(&A) -> bool) -> bool;
}

pub trait AdditiveIterator<A> {
    fn sum(&mut self) -> A;
}

pub trait MultiplicativeIterator<A> {
    fn product(&mut self) -> A;
}

pub trait OrdIterator<A> {
    fn max(&mut self) -> Option<A>;
    fn min(&mut self) -> Option<A>;
}
```