Add sample-based generation example: synth_tones

[Wandalen]

It is refactored version of the example beep. I bubbled up a callback to generate a sample. Also, I replaced unwrap with proper handling of errors. That does not add complexity because anyhow and ? are already in use. That's the first step I would like it was done for me to start playing with cpal and integrating it into my projects.

Hopefully, it will be useful for not only me.

#612