Mixer v2 + fix for memory leak

[MarkKremer]

• Add an option to Mixer to let it drain when all its streamers have been drained instead of playing silence. I have added this in preparation of an optimization that I would like to do in the speaker to auto-close Oto's player when nothing is being played anymore. edit: may not be relevant anymore.
• Updated the Streamer interface. Now it allows Streamers to use the samples passed to Steam() as scratch space even if it doesn't fill it completely. Mixer previously didn't adhere to the interface when it said it wasn't allowed to do that. I think programs can be more efficient when Streamers are allowed to do that. This is also how the Reader interface works. Reverted this change to keep backwards compatibility. I may introduce this change again when we move to v3.
• Use Mixer to replace the implementation for Mix(). Mix() should work the same as before.
• Fix memory leak where drained streamers weren't being garbage collected.

Edit:
Resolves #179