Add mixer::Chunk::from_wav.

This is a bit involved patch, since there are now two different chunk representations (one allocated/borrowed from SDL functions, one owned directly by Rust).

A side effect is that `Chunk` fields are no longer accessible from outside, which is actually desirable as updating the buffer while it is being played is not good. I guess we can add an accessor to `alen` however, which should be in another PR.