Get most basics working

- Control (at least) 1 neopixel
- Add basic blinky panic handler
- Support N-key rollover
- Test GPIO pins

src/neopixel.pio

@@ -0,0 +1,11 @@
+.program ws2812
+.side_set 1
+.wrap_target
+bitloop:
+   out x 1        side 0 [6]; Drive low. Side-set still takes place before instruction stalls.
+   jmp !x do_zero side 1 [3]; Branch on the bit we shifted out previous delay. Drive high.
+ do_one:
+   jmp  bitloop   side 1 [4]; Continue driving high, for a one (long pulse)
+ do_zero:
+   nop            side 0 [4]; Or drive low, for a zero (short pulse)
+.wrap