Rust definitions for mmal

This is the library used on the Raspberry Pi to interact with VideoCore. Basic functions are to control the camera, encode and decode video. Applications that use it (natively, not via Rust) include raspistill and VLC.

These FFI definitions are, for the most part, automatically generated by bindgen.

Please note that I have no idea what I am doing with rust so bindgen could probably be tweaked to improve its output.

PRs welcome. Breaking changes that make the library better to use eg changing constified_enum_module are also welcome.

Steps for improvement:

• Evaluate if rust nightly is useful or switch on/off bindgen's nightly flag if running in nightly or not
• Make API as natural as possible
• Manually add MMAL_ENCODING constants and similar that aren't supported by bindgen
• Implement fmt::Display trait for more types or remove it entirely