Provide memory buffer -> mixer Chunk conversion methods

[jstasiak]

This basically provides an API to access Mix_QuickLoad_WAV and
Mix_QuickLoad_RAW with one caveat – thie API takes ownership of the
buffers, while Mix_QuickLoad_* functions don't. Since Rust-SDL2 wraps
Mix_Chunks in its own structure with its own buffer ownership flag it's
easy to work around that and deallocate the buffer when Chunk is
dropped.

There are few design considerations here:

• Taking ownership of the buffer. I couldn't come up with a way to
  get a Chunk based on a buffer while still keeping the code safe and
  making sure that as long as Chunk is alive the buffer is also alive
  and not making the change be terribly involved and disruptive
• What's the best way to take an ownership of a buffer passed as a
  parameter? I came up with Box<[T]> as it seems universal enough,
  but I expect a more idiomatic Rust way may exist.

I modified the mixer demo to show how the API is used, a beep is
generated when no sound file is passed to the example.

[jstasiak]

Actually, I'm not sure if integrating Mix_QuickLoad_WAV is feasible – there's no way to pass buffer length to it and it performs blind buffer traversal (and reads buffer length from the buffer itself) on the assumption that the contents are correct. If they're not, we'll get a segmentation fault or even possibly something else. I'm considering dropping it, as loading from raw buffer seems much more useful to me anyway.

[jstasiak]

I removed the Chunk::from_wav_buffer() method from this pull request following the earlier comment, it can be introduced at a later point if necessary/feasible.

[jstasiak]

Is there something I can do to get this merged? If this is inappropriate to be included in Rust-SDL2 just let me know.

[Cobrand]

Sorry I totally forgot about it, LGTM so I'll merge this.

[jstasiak]

Cheers!