Adds whitenoise synth

[pedro823]

This branch also cleans up a lot of code.

whitenoise is a synth that produces white noise with an envelope that lasts
in full volume for how long sus is. That is, it has no decay. Its pitch can
be adjusted with the frequency parameter, since it uses a band pass filter to
only allow certain frequencies through.

How can this be tested?

• Clone this branch.
• run python3 setup.py install.
• run python3 -m FoxDot.
• try it out:

# since sus == dur, a new noise will always play when the other is leaving,
# making a constant sound
d1 >> whitenoise(0)

# to change this, try making sus < dur
d1 >> whitenoise(sus=1/2)

# Changing pitch
d1 >> whitenoise(20, sus=1/2)

# Very good for percussion!
d1 >> play('x ', lpf=500, lpr=1.1)
d2 >> whitenoise([25], amp=0.2, dur=[rest(5/2), 1/2, rest(1)])
d3 >> play('       s', hpf=3000)