Add a 20V darlington source. Switch to STM32F042 (Schematic changes only)

This provides *both* the low current zener noise source and a new 20V
darlington noise source to see which one works better.

The low current zener source has been updated to filter the input
power and provide for both low-pass filtering and configurable gain in
both op-amp stages to allow for more experimentation.

Signed-off-by: Keith Packard <keithp@keithp.com>