Improve timbre syntax and fix silence and tone.
Fix fern test.
Only one note on a frequency at a time.

This makes it easier to make an immediate-mode piano using the
turtle.tone(freq) command.  turtle.tone(freq, false) stops the sound.
Also, it helps solve the "too many oscillators" problem by stopping
oscillators when others of the same frequency have been started.
Add a Piano sprite.

This sprite sounds like a piano instead of a square wave when played,
and it also looks like a piano, lighting up keys when notes are played.
Things interact fine with both play() and tone().

The default turtle sounds is still a square wave: to get a piano, you
need to say p = new Piano, then p.play 'edc'.