Add ChannelLayout API (#38)

* Create channel_layout directory

* Rename ChannelLayout -> ChannelLayoutMask

* Add enum Channel (<-> AVChannel)

* Add struct ChannelCustom (<-> AVChannelCustom)

* Add enum ChannelOrder (<-> AVChannelOrder)

* Add struct ChannelLayout

- Smart copy-on-write pointer to AVChannelLayout
- idiomatic Rust API wrapping around FFmpeg APIs
- no Ambisonic support (yet)
- consts will need to be manually updated

* Add ChannelLayoutIter (iterator over all standard layouts)

* Add codec/Audio::ch_layouts

* Add ch_layout API to Audio-related structs

examples/transcode-audio.rs

@@ -91,7 +91,7 @@ fn transcoder<P: AsRef<Path>>(
     let channel_layout = codec
         .channel_layouts()
         .map(|cls| cls.best(decoder.channel_layout().channels()))
-        .unwrap_or(ffmpeg::channel_layout::ChannelLayout::STEREO);
+        .unwrap_or(ffmpeg::channel_layout::ChannelLayoutMask::STEREO);
 
     if global {
         encoder.set_flags(ffmpeg::codec::flag::Flags::GLOBAL_HEADER);