* Fix pulseIn() on SAMD51 by writing it in plain C, so that
the CM0+-specific pulse_asm won't need to be linked.
The SAMD51 is fast enough that we can time pulses with micros()
Tested with a bunch of pulse lengths from <1us to >1s
* Implement a new delayMicroseconds() function for SAMD51
This version enables the "Debug Watchpoint and Trace" module (DWT)
in startup.c and then uses the 32bit cycle counter that is part
of DWT to count cycles indepenent of instruction timing.
Tested for good accuracy with various values between 1 and 2000us.
