Implement imread() and imwrite() #4
Description
Should just read and write image files from and to the MicroPython file system.
Unclear if we can just call the C++ functions, and whether they will somehow magically know how to access the MicroPython file system. Probably not... In which case, probably need to use the MicroPython VFS API to actually read and write files to flash, then use OpenCV's imdecode() and imencode() to convert between raw file bytes and Mat objects.