In a test this improved speed substantially, nearly doubling
the speed of the following test program
```python
pixels = neopixel.NeoPixel(pixel_pin, pixel_num, brightness=1, auto_write=False, pixel_order="RGB")
evens = helper.PixelMap(pixels, [(i,) for i in range(0, pixel_num, 2)], individual_pixels=True)
animation = RainbowChase(evens, 0, spacing=8)
t0 = adafruit_ticks.ticks_ms()
while True:
for i in range(10):
animation.animate(show=False)
t1 = adafruit_ticks.ticks_ms()
print(f"{10000/(t1-t0):.0f}fps")
t0 = t1
```
Performance on Raspberry Pi Pico W:
Before: ~85fps
After: ~140fps
This also happens to make it compatible with an in-process PR that adds
a fast PixelMap-like class to the core, but which doesn't support
getitem.
