A signal is like a pin, but ca also be inverted (active low). As such, it
abstracts properties of various physical devices, like LEDs, buttons,
relays, buzzers, etc. To instantiate a Signal:

pin = machine.Pin(...)
signal = machine.Signal(pin, inverted=True)

signal has the same .value() and __call__() methods as a pin.

Allows to translate C-level pin API to Python-level pin API. In other
words, allows to implement a pin class and Python which will be usable
for efficient C-coded algorithms, like bitbanging SPI/I2C, time_pulse,
etc.

Should work on any machine that provides the correct pin functions.

Move definition of mp_builtin_open_obj to a separate module, then file.c
becomes more or less compartmentalized FatFs file class, which can be used
together with file class implementations for other (V)FSes.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.