consider making the iterator tutorial into the std::iter module docstring

[thestinger]

I'm not sure if we should do this, but it would certainly make it easier to discover.

[alexcrichton]

I would be in favor of this.

[brson]

+1

[thestinger]

I think I might wait until we're extracting/testing code samples in doc strings because I don't want them to break.

[adrientetar]

@thestinger Will you send a patch for it?

[pnkfelix]

part of #11755

[steveklabnik]

The 'iterators tutorial' is part of the containers guide: http://doc.rust-lang.org/guide-container.html#iterators