Stackless support, new 16-bit PIC port.

py core:
- builtin round() accepts second arg
- add more special methods: __pos__, __neg__, __invert__
- support for stackless Python calls in VM, strict and non-strict
- support for object representation that suits 16-bit archs
- implement closures in native code emitter
- implement str.splitlines() method

lib:
- fatfs library upgraded to R0.11 (fixes bugs with unlink)

pic16bit port:
- new port to 16-bit PIC (reference MCU is dsPIC33J256GP506)
- heap is 4600 bytes
- implement basic pyb module with delay
- implement basic LED and Switch classes

docs:
- initial docs for micropython module
- additional example for Timer callback usage
- document instantiating struct objects in uctypes