Migrate crates to Rust edition 2021 (#24)

* Migrate ffmpeg-sys to Edition 2021

* Migrate ffmpeg to Edition 2021

* Remove now-redundant imports

* Reorder imports after edition migration

examples/remux.rs

@@ -2,7 +2,7 @@ extern crate ffmpeg_the_third as ffmpeg;
 
 use std::env;
 
-use ffmpeg::{codec, encoder, format, log, media, Rational};
+use crate::ffmpeg::{codec, encoder, format, log, media, Rational};
 
 fn main() {
     let input_file = env::args().nth(1).expect("missing input file");