Starting Teensy 4 demo sketches for beta

EchoBoth - reads Serial1 to USB for debug on early T4 beta. Feed Serial1 Rx (of another Teensy) from T4's Tx pin #17 - setup for debug in beta

BlinkMin - No delay blink with startup timing

--- output vals for reference USB :: Serial First millis=797 and !Serial wait loops :: while Count ii=1311696 debug :: Into setup() > millis=306 and Loop Hz 21394930

Blink_IntvTime - Running IntervalTimer and streaming strings out Serial USB 64 bytes per loop() or 1587136 bytes/sec

--- output vals for reference on Serial USB rows of alpha chars, with Z row pushed out with tab char Count Interval Timer _isr ITcnt=498147 and passes through loop and running micros loop() ii=24799 micros=12130230