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

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [0.5.3] -
 ### Changed
+- Update to 0.19.0 version of rust-bindgen; rebuilds are much faster.
 - gadomski: add `set_option()` method to wand API.
 - gadomski: add `write_images_blob()` to create animated GIFs.