Auto merge of rust-lang#43077 - SimonSapin:ranges, r=alexcrichton

Implement O(1)-time Iterator::nth for Range*, and slim the Step trait

Fixes rust-lang#43064.
Fixes part of rust-lang#39975.
Fixes items 1 <s>and 3</s> of rust-lang#42168.
CC rust-lang#27741.

I think rust-lang#42310 and rust-lang#43012 should not have landed without the `nth` part of this PR, but oh well.

Author: bors