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.
The idea behind decrease is: bytecode and other static data is also kept on
heap, and can easily become half of heap, then setting threshold to half of
heap will have null effect - GC will happen on complete heap exhaustion like
before. But exactly in such config maintaining heap defragmented is very
important, so lower threshold to accommodate that.
To start with, the critical scripts _boot.py and flashbdev.py are frozen
to improve performance and reduce RAM consumption.
Saves about 1000 bytes of heap RAM for a bare boot with filesystem.
