identify silences (inefficiently)

This seems like it might be a much better
first-pass "note split detector". It
identifies the most important splits with
likely very high accuracy.

There are other important splits that it
likely _won't_ catch (switching frequencies
while keeping amplitude) but those are
actually possible to manually avoid while
recording.

This is a naïvely coded proof-of-concept,
it's currently needlessly expensive.
trying out another noise-cleaning approach

This does _very_ well on melodies that have
clear separation between notes, i.e. no
"sliding".

(For instance, it gets near-perfect results
when whistling the Final Fantasy victory
tune.)

I think it might be better to build on this,
and expand it to detect the somewhat rarer
case of notes changing frequency without
even a brief period of silence.