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

README.md

@@ -7,6 +7,8 @@ A somewhat safe Rust interface to the [ImageMagick](http://www.imagemagick.org/)
 * Rust
 * Cargo
 * ImageMagick (version 6.9 or higher)
+* [rust-bindgen](https://github.com/Yamakaky/rust-bindgen) (version 0.19 or higher)
+    - This will be installed automatically if it is missing.
 
 See the `docs/Development_Setup.md` file for details particular to each platform.