These drivers can now be used by any port (so long as that port has the
_onewire driver from extmod/modonewire.c).
These drivers replace the existing 1-wire and DS18X20 drivers in the
drivers/onewire directory. The existing ones were pyboard-specific and
not very efficient nor minimal (although the 1-wire driver was written in
pure Python it only worked at large enough CPU frequency).
This commit brings backwards incompatible API changes to the existing
1-wire drivers. User code should be converted to use the new drivers, or
check out the old version of the code and keep a local copy (it should
continue to work unchanged).
The OneWire class is now in its own onewire.py module, and the temperature
sensor class is in its own ds18x20.py module. The latter is renamed to
DS18X20 to reflect the fact that it will support both the "S" and "B"
variants of the device.
These files are moved to the modules/ subdirectory to take advantage of
frozen bytecode.
