OpusEncoder and OpusBufferedEncoder Interfaces

[mattgwwalker]

OpusEncoder is currently the base class for OpusBufferedEncoder.

Naturally, they should both have an encode() method. However the two classes return different results. OpusEncoder returns a single encoded frame, while OpusBufferedEncoder may return multiple encoded frames.

Their interfaces need to be tidied up. Perhaps the direct-inheritance should be replaced with a common base class.