Add a generic Chain iterator

[rust-itertools]

Add a generic Chain iterator

[bluss]

If it works for general iterators, it will work for the mutable-reference carrying ones as well.

[bluss]

I'm unsure if this is needed. It's not such a big thing as .zip(), since chain doesn't need to flatten anything. Chaining .chain() calls should work fine.

[Robbepop]

To me it would be a superior syntax in cases where I need to chain more than two iterators together. But you are right that a Chain iterator isn't as needed as a Zip iterator was.

[bluss]

I will decline this up front for now. Thanks anyway.

Supporting "n-ary" constructions in Rust just requires hacks, traits over several tuple types and so on. In this case it is not worth it (does not buy us enough), and the regular chaining style is truly n-ary for any n, not just a finite trait implementation list.