Rethinking the beep.Streamer interfaces

[MarkKremer]

• Removed beep.StreamCloser and beep.StreamSeekCloser. These functions do not seem to add much value because the decoders that implement them only proxy the call to their source io.Reader. Besides, other beep.Streamers do not call Close() on their source streamers, so the user has to keep track of when to close the decoder or reader themselves regardless. The result of this is that all io.Reader's padded to a Decode() function must be closed manually.
• midi.NewSoundFont() also doesn't close the supplied io.Reader anymore.

TODO

• Test all the examples
• Review docblocks
• Update wiki

[dusk125]

2 cents:
I think we should keep the interfaces, in the examples, they generally open the file, pass it to the decoder, and play it; closing the backend resource at scope end is fine if the lifetime of the frontend streamer is less than or equal to the backend resource.

However, I could see usecases where we want to pass the ReadCloser into a streamer and keep the streamer around outside of the calling scope (which in the new examples would free the underlying resource and cause a panic).

I think that keeping the StreamCloser interface is good and we could make it a bit smarter such that the decoder just takes a Reader, then checks to see if it's a *Closer when decoder.Close is called, otherwise, it's a no-op.

If that makes sense.
What do you think?

[MarkKremer]

You make some good points. My argumentation for this interface is that Beep's composites need to be configured separately anyway. E.g. if you have a streamer, volume and ctrl node, you'll need to keep hold of them so that you can close, change volume and pause the stream. I think it's a good programming pattern to keep them in a struct to encapsulate that part of the audio business logic. So what's an extra io.ReadCloser? Removing Streamer.Close may clear up some confusion as to if both streamers and files must be closed.

I invite further opinions but I'll probably hold this PR until after I'm further with the docs and/or my music player projects so I'll have more experience.