Using the callback to take action on an encoder turn was initially
optional, but since it is much more efficient to use interrupts and
callbacks rather than polling this has been made mandatory. It is still
possible to manually tell the Encoder class to poll rather than use
interrupts, but for consistency this will still call the callback.

Also tried to make some minor performance improvements.

Using interrupts is easy in MicroPython, and it makes sense to do so for
a rotary encoder - this commit introduces the ability to specify a
callback to the Encoder class, which will be called when a rotation is
detected.

Initial commit of a rough copy of the 'uEncoder' library. This is based
on the existing C++ at https://github.com/seanlano/WiLED/blob/\
implement_WiLP/libraries/Encoder/Encoder.cpp.