Use new rust-bindgen crate for faster builds

Rather than cloning the bindgen repository and building from source, use
the packaged crate. The result is the rebuilds are much, much faster.
Also, had to make numerous changes for the new version of bindgen.

cargo test passes

src/lib.rs

@@ -34,7 +34,7 @@ extern crate libc;
 mod wand;
 mod conversions;
 pub mod filters;
-mod bindings { include!(concat!(env!("OUT_DIR"), "/bindings.rs")); }
+include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
 
 pub use wand::*;