Implement std::marker::Send for ears::SoundData

[humb1t]

Dear @jhasse can you make SoundData sendable between threads. Right now it's possible to move music into thread, but not a sound:
the trait boundstd::rc::Rcstd::cell::RefCellaudio::music::ears::SoundData: std::marker::Sendis not satisfied [E0277]

[jhasse]

I actually don't know how to do that. Haven't looked that much into multithreading with Rust.

[humb1t]

Ok, I will try to submit a solution