Numerous improvements, bug fixes, and enhancements

Thanks to @Marius-450 for many improvements, including:

• memory improvements : smaller background bitmap scaled to fullscreen and added a scale parameter to scale the "canvas".
• colors : +7 colors (for same memory consumption), background color management.
• mode is now implemented and consistent with documentation.
• Images : added bgpic() to display a bitmap as background, using OnDiskBitmap. Added changeturtle() to change the turtle shape via OnDiskBitmap or by passing a tilegrid object.
• Implemented : pensize(), speed(), reset() and some helpers like distance() or towards()
• Stamping of the turtle.
• Some details like fixing rounding errors in circle(), or improving dot()

To use in CircuitPython, simply install the Adafruit CircuitPython bundle.

To use in CPython, pip3 install adafruit-circuitpython-turtle.

Read the docs for info on how to use it.