Add 9bit support to HardwareSerial

[svatoun]

I stumbled over a few different implementations / tweaks how Arduino could read 9bit data using Serial objects; the core library does not support 9bit, while the standard UART HW does ...

I am aware of issues arduino/Arduino#2292 and arduino/Arduino#2291 - however I'd like to take a differen timplementation path: use guard macro to conditionally enable the extra code (and twice as long buffer). This way, normal user will not see any performance/size penalty, but developer who needs 9bit support could enable it at the documented cost.

Diffs will be attached shortly as PR...