unix/modos: Add uos.rename and uos.rmdir.
unix/modos: Add uos.rename and uos.rmdir.

The existing uos.remove cannot be used to remove directories, instead
uos.rmdir is needed.  And also provide uos.rename to get a good set of
filesystem functionality without requiring additional Python-level os
functions (eg using ffi).

MrSurly/micropython,kerneltask/micropython,tobbad/micropython,MrSurly/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,pramasoul/micropython,bvernoux/micropython,MrSurly/micropython,selste/micropython,tobbad/micropython,kerneltask/micropython,henriknelson/micropython,tobbad/micropython,kerneltask/micropython,kerneltask/micropython,tobbad/micropython,pozetroninc/micropython,selste/micropython,pozetroninc/micropython,adafruit/circuitpython,henriknelson/micropython,selste/micropython,pramasoul/micropython,pramasoul/micropython,bvernoux/micropython,tobbad/micropython,pramasoul/micropython,selste/micropython,henriknelson/micropython,henriknelson/micropython,MrSurly/micropython,pramasoul/micropython,adafruit/circuitpython,adafruit/circuitpython,kerneltask/micropython,bvernoux/micropython,bvernoux/micropython,bvernoux/micropython,pozetroninc/micropython,pozetroninc/micropython,pozetroninc/micropython,adafruit/circuitpython,MrSurly/micropython,selste/micropython