Hide use std::fmt; in all examples in '_, the anonymous lifetime

[rusty-snake]

https://doc.rust-lang.org/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html

// Rust 2015

use std::fmt;

fn make_wrapper(string: &str) -> StrWrap {
…

// Rust 2018

fn make_wrapper(string: &str) -> StrWrap<'_> {
…

In the 2018 example is use std::fmt by default hidden, in 2015 not. (looks inconsistent)