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

https://doc.rust-lang.org/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html

```rust
// Rust 2015

use std::fmt;

fn make_wrapper(string: &str) -> StrWrap {
…
```

```rust
// Rust 2018

fn make_wrapper(string: &str) -> StrWrap<'_> {
…
```

In the 2018 example is `use std::fmt` by default hidden, in 2015 not. (looks inconsistent)