Nudging users off 2015 edition

There are real costs to having 2015-edition Rust source code, and arguably no benefits. I think rustc and Cargo should start warning about crates still using the 2015 edition and encourage moving to a newer edition. users who try to use rustc directly get caught by the edition 2015 default, and find Rust being "weird" and rejecting code that works in Cargo and the Rust playground. Even tough invoking rustc directly isn't the best way to get started with Rust, people coming from C expect this ...