esp8266/modules: Split onewire.py into OneWire and DS18X20 driver.

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.