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

src/util/media.rs

@@ -1,5 +1,5 @@
-use ffi::AVMediaType::*;
-use ffi::*;
+use crate::ffi::AVMediaType::*;
+use crate::ffi::*;
 #[cfg(feature = "serialize")]
 use serde::{Deserialize, Serialize};