SPI DMA / STM32 Neopixel output

[gfwilliams]

Previously we disabled IRQs during the Neopixel sending, but that could cause lost serial data.

Now, re-enabling them might cause glitches - although less likely now that all data to be sent is in one big, flat buffer.

Need to check this (for 1v94) and potentially move to a DMA-based SPI send if needed.

[gfwilliams]

Best solution would be to use DMA for SPI with an optional callback (to allow async transmission). Larger strips of lights could have the bit patterns generated as needed.

Ideally a jshSPISend with a buffer, as per #695

And then hopefully this could even be exposed as an SPI.writeAsync for http://forum.espruino.com/conversations/257857