Hardware Uart ISR not Shareable

[Makuna]

Problem Description

The Hardware Uart support shares a single interrupt between both Uart0 and Uart1. Current implemementation does not allow for it to be shared so that Serial is usable at the same time that features of Uart1 can be used for other purposes, like updating NeoPixels (Ws2812 and the like).

@unaiur has created this pull in an attempt to solve the issue. Unfortunately the Uart code has changed significiantly and this pull needs work before it can be merged.

There doesn't seem to be an original issue that pull was being applied too.

[Makuna]

NeoPixelBus has a branch created that uses these changes
https://github.com./Makuna/NeoPixelBus/tree/SharedUartIsr