Audio system and asset rework

[sornas]

Well, this took too long and got too big. The audio part is far from finished but at this point I'd rather get the infrastructure merged.

Audio

• Load f32 48000Hz stereo WAV.
• Play multiple sounds and the same sound concurrently.
• Reload sound assets.

Assets

• Add asset IDs
• Get loaded assets via indexing. (See main.rs)

[sornas]

If the API looks Ok I can add some documentation before merging.