pijul/*: reorganize imports and remove extern crate

I personally find it easier to mentally parse imports when they are separated into blocks of std imports, external crate imports, and internal crate imports.

Additionally, #[macro_use] extern crate is a Rust 2015-ism, and can be replaced by useing the macro (derive or procedural).

Created by  cole-h  on November 19, 2020
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