Make a new `beat_speed_modulated` example

[rand00]

Copy the beat example to a new beat_speed_modulated example-directory, and add another beat in it by calling Fry.Beat.Make (call the returned module Beat_speed_mod).

Then use Beat_speed_mod.s with S.map (and e.g. the sin function) to modulate the speed of Beat by passing the signal as bpm_s.