Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].

Fix adc to work with resolution changes.

Added support for the ADC channels and mappings to make_pins.py

I'm not sure if the hal properly deals with the channel 16/18 differences
between the 40x and 42x. It seems to deal with it partially. This particular
aspect will need testing on a 42x or 43x.

Late USB enumeration could clear settings after they had been set.
Now fixed by not clearing some settings on init.

RX was blocking if received characters were not being processed, so
CTRL-C would not be picked up.  Now "fixed" by not blocking, but
instead discarding incoming characters if they overflow the buffer.

As part of this, rejig the way TIM3 is initialised, since it's now
shared by USB CDC and the blue LED PWM.

This gives a functioning, independent MSC device.

Just low-level read/write support.  No filesystem yet.