Replace usage of `extern crate` with just `use` as needed as of Rust 2018 Edition.

[dconnolly]

With only a few exceptions.

https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html#no-more-extern-crate

[zancas]

It is because of this convention that I approach new crates with the rule of thumb:

Step One: Examine the "Cargo.toml".

This seems like healthy behavior to me.