Phase1a of audio engine rewrite

[yara-blue]

supersedes #902

Tracking issue: #901

Very minimal still, to get an idea of what this will all look like see: rodio-experiments.

Implementation notes:

• I've moved some documentation into md files and include_str! those to prevent duplication. We'll be using that even more for the effects.
• For now does not rename Source but do make it available under it's new name via a pub re-export in lib.rs
• There is a placeholder so the example in the docs works