MS20-style VCF

From the schematic at https://www.schmitzbits.de/ms20.html with a couple of small tweaks to allow for high/low pass to be toggled.

This module has a bunch of issues! The pots for CV control are wired backwards, so they are fully "open" when at the "low" position. If I ever do a second run, the kicad schematic has a bunch of notes about what to fix....