Add foreach to IteratorExt

[larroy]

Its a common idiom in other functional languages for side effects when chaining iterator functions

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

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[nikomatsakis]

As discussed on IRC, I think that an addition like this to a core library ultimately requires an RFC, so I'm going to close this PR for now. My own personal feeling is that a for loop is a nice way to do side-effects, but I can certainly see the appeal of foreach as an alternative in some cases.