Add 'assign to slice' (__setitem__) method, plus various speed improvements

[gsmith-to]

This is a mostly a number of small changes to improve efficiency; and there are two 'feature adds':

• you can now assign to a slice, e.g. pixels[:12:2] = red, see examples/set_range.py
• rotate_left() and rotate_right() can now be called with no parameter; defaults to 1 (this looks like it was the original intent)

I had looked for a way to pass a range of pixels to set_pixel, so that when calling set_pixel_line or fill, we could avoid the repeated calculation inside set_pixel; instead, just do it it once and store it multiple times. By making set_pixel accept a slice object for 'pixel_num', we get this advantage and can also implement the __setitem__ operation for assigning to slices, by just forwarding the slice to get_pixel.

The branch is a series of small, mostly independent mods (one per commit) to make it easier to review; and relatively easy to drop any you don't like.

NOTE: I have only tested with "GRB" mode, since that's what the LEDs I have are.

[blaz-r]

Amazing work! I tested with GRB and RGBW and it all seems to work. The improvement of efficiency is quite significant. Thanks a lot for all this. 😄