Updated TeensyDMX library for Teensy 4

[shawn]

I've updated the TeensyDMX library to support Teensy 4.

For now, please see the `teensy4` branch at: https://github.com/ssilverman/TeensyDMX/tree/teensy4
I'll be merging it into the master branch once it's settled. It's a little bit ahead of 4.0.0-alpha.2.

Note that it has some extra features and a few minor differences from the 3.x.x versions. Also note that this branch still supports all the Teensys 3 and later: 3.1, 3.2, 3.5, 3.6, LC, and 4.0.

Comments and suggestions welcome.