Do a bunch of clean up and add some comments

README.md

src/bindgen/annotation.rs

src/bindgen/cargo_expand.rs

src/bindgen/cargo_toml.rs

src/bindgen/cdecl.rs

src/bindgen/config.rs

src/bindgen/items.rs

src/bindgen/library.rs

src/bindgen/mod.rs

src/bindgen/utilities.rs

src/bindgen/writer.rs

src/main.rs