Return with value gets converted to StopIteration(value). Implementation
keeps optimizing against creating of possibly unneeded exception objects,
so there're considerable refactoring to implement these features.

py/objstr.c: Implement str.rfind() and add tests for it, refactor find_subbytes().

They still exist in commented-out form in objexcept.c if they are ever
needed.

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

Add mp_obj_module_register

Some changes that I forgot to push with my last pull request

Fix adc to work with resolution changes.

* Add function to load static modules.
* Use module_register to pyb module.

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.

Using pyboard.py you can use Python running on your PC to execute
commands on the connected pyboard.  See examples in that file.

py: Implement support for "except Exception as var" clause.

For this, needed to implement DELETE_NAME bytecode (because var bound
in except clause is automatically deleted at its end).
http://docs.python.org/3/reference/compound_stmts.html#except :
"When an exception has been assigned using as target, it is cleared at
the end of the except clause."

Based on issues raised during recent review and inconsistency of different
implementations.

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.

py/builtinmath.c: use tgamma() instead of gamma().