Add Iterator::into_vec()

[leonardo]

I've written some comments in the closed thread.

This feature goes according to:

[Roadmap 2017] Productivity: learning curve and expressiveness

Compared to all the times I had to code review “you should be moving that shared_ptr<> in C++”, I really like Rust’s existing choices here. Copy when it’s trivial enough to not care, move by default so the right thing usually happens without thinking, but also really easy to .clone() when you have to. (I needed to, essentially, .clone() a SqlParameter in C# the other day, and it was miserable. C#'s IClonable totally doesn’t work, and canonical C# doesn’t reliably even have a “copy constructor…