Originally, .methods was used for methods in a ROM class, and
locals_dict for methods in a user-created class.  That distinction is
unnecessary, and we can use locals_dict for ROM classes now that we have
ROMable maps.

This removes an entry in the bloated mp_obj_type_t struct, saving a word
for each ROM object and each RAM object.  ROM objects that have a
methods table (now a locals_dict) need an extra word in total (removed
the methods pointer (1 word), no longer need the sentinel (2 words), but
now need an mp_obj_dict_t wrapper (4 words)).  But RAM objects save a
word because they never used the methods entry.

Overall the ROM usage is down by a few hundred bytes, and RAM usage is
down 1 word per user-defined type/class.

There is less code (no need to check 2 tables), and now consistent with
the way ROM modules have their tables initialised.

Efficiency is very close to equivaluent.

Addresses issue #377.

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.