Use the rust-bindgen crate properly

Using the changes from @gadomski along with some additional work, to get the
generated bindings working again. Works on macOS and FreeBSD 11. A couple of
hacks are needed for FreeBSD, but nothing too serious.

Changed to use the libc prefix, and changed to use the generated enums.

Fixes #22, #15, and #14

cargo test passes

docs/Development_Setup.md

@@ -13,7 +13,7 @@
 $ xcode-select --install
 $ ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
 $ brew install rust
-$ brew install imagemagick
+$ brew install imagemagick@6
 $ brew install pkg-config
 ```