BUG(2483): Only export public symbols.
OTHER: Consistent #include style.
OTHER: Remove bundled opusfile since it's released now.
OTHER: Update MIME type of Opus to recommended.

From https://wiki.xiph.org/OggOpus#Content_Type:

The recommended mime-type for Ogg Opus files is audio/ogg,
defined in RFC 5334.

If more specificity is desired, one can distinguish Opus
files as 'audio/ogg; codecs=opus'.

The recommended filename extension for Ogg Opus files is .opus.