The convention is to always keep `extern crate`s in the crate root. `use`s get moved around, but `extern crate`s stay put. Think of `extern crate` as if it were

The convention is to always keep `extern crate`s in the crate root. `use`s get moved around, but `extern crate`s stay put. Think of `extern crate` as if it were a crate-internal `pub use` (because that's kind of what it is!).