Eurorack CMOS logic ADSR envelope generator.

• adsr2 - original version, based on separate switch ic and demultiplexer. (this one has known design bugs).

• adsr2v2 - use a dual switch multiplexer/demultiplexer for everything. Should work, but will likely not be prototyped.

• adsr2v3 - improved v2, use a 40106 buffer/inverter for IO buffers instead of transistors (=lower component count). Prototype build looks and works exactly as intended. :)