std::iter: Introduce .by_ref() adaptor

std::iter: Introduce .by_ref() adaptor

Creates a wrapper around a mutable reference to the iterator.

This is useful to allow applying iterator adaptors while still
retaining ownership of the original iterator value.

Example::

    let mut xs = range(0, 10);
    // sum the first five values
    let partial_sum = xs.by_ref().take(5).fold(0, |a, b| a + b);
    assert!(partial_sum == 10);
    // xs.next() is now `5`
    assert!(xs.next() == Some(5));

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This adaptor requires the user to have good understanding of
iterators or what a particular adaptor does. There could be some
pitfalls here with the iterator protocol, it's mostly the same issues
as other places regarding what happens after the iterator
returns None for the first time.

There could also be other ways to achieve the same thing, for
example Implementing iterator on `&mut T` itself:
`impl <T: Iterator<..>> Iterator for &mut T`  but that would only
lead to confusion I think.