Upstreaming to the standard library

[Amanieu]

Hello itertools users and maintainers!

The Rust library team has recently discussed the possibility of upstreaming some of the more commonly used itertools methods to the standard library.

We now have a proper solution to naming conflicts via supertrait item shadowing which are hoping to stabilize soon. This will cause name resolution conflicts between Iterator and Itertools to always resolve in favor of the sub-trait (Itertools) in the case of an ambiguity, which should keep all existing code working correctly.

We would like feedback from maintainers and users of itertools as to which methods are the most commonly used and would be most suitable for upstreaming.