Allow an Iterator to update a &mut [T], add IterReader

[erickt]

This PR adds two features to make it possible to transform an Iterator<u8> into a Reader. The first patch adds a method to mutable slices that allows it to be updated with an Iterator<T> without paying for the bounds cost. The second adds a Iterator adaptor, IterReader, to provide that Reader interface.

I had two questions. First, are these named the right things? Second, should IterReader instead wrap an Iterator<Result<u8, E>>? This would allow you to IterReader::new(rdr.bytes()), which could be useful if you want to apply some iterator transformations on a reader while still exporting the Reader interface, but I'd expect there'd be a lot of overhead annotating each byte with an error result.

[huonw]

update could theoretically work with any Iterator<&mut T>, right?

[alexcrichton]

Would something like this suffice?

fn update<I: Iterator<T>>(self, iter: I) -> uint {
    let mut i = 0;
    for (slot, element) in self.mut_iter().zip(iter) {
        *slot = element;
        i += 1;
    }
    return i;
}

If the unsafe blocks stays, could you add a comment explaining why it is unsafe, what the benefits are, and why it is necessary to not rely on safe code?

[erickt]

You're right, that could work. I'll remove that patch from the PR.

[alexcrichton]

The first patch does seem to deviate a bit from the compositionality of iterators. This would add an update method to vectors, but not to all the other containers that we have? I think that @huonw's suggestion of working with Iterator<&mut T> and Iterator<T> would indeed maintain composing this with other types as well.