A couple of minimalist changes.

systick_isr - By default it does nothing with the event responder.
Only if the user calls the attachInterrupt member of EventResponder will it install the version of the ISR that checks the list for any active ISR event Responders.

T4 Yield - try to keep bitfields for things yield should test
Make it such that yield hopefully can test to see there is nothing to do and returns quickly.

There are some interesting limitations, that is that the serialEvent
handling code. Both for USB and hardware Serial can only remove it self
if the default eventHandler is called once, as I don't know anyway to
detect if the default code (weak linkage) is included or if it is a user
version.  So waits until called to remove it self from active list.
Sketches can force this by simply calling the event method right after
calling begin on the serial port.

Add recommended 24MHz crystal setting

Later, we should check if we can reduce the bias, also (For me, it works with minimal bias -but I think should should keep it for now, as it is).

There were some issues in the setRX and setTX functions for T4,x

fixes #464

It never came up much with T4 as there were not really alternative pins...

But fixed and tested with simple sketch on T4.1 and simple jumper to TX and RX pins on memory chip locations

T4.x CTS pins - T4.1 add memory chip area to Serial ports.

A few of the T4 and now T4.1 Serial ports support a CTS pin.  One was previous fixed, added the others to tables.

Also T4.1 has Some Serial port pins in the area that the optional memory chips could be installed

fix double promotion

MPU: Trap not allowed accesses

Arduino-Compatibility: digitalWrite controls pullup/pulldown in inpu…

RX with 4 transfers using similar code to seremu

usb.c / interrupt - callbacks use ctz()

fix descriptor for teensy 4 usb audio midi serial

https://forum.pjrc.com/threads/60174-Understanding-The-USB-Specification

Fix Macro clockCyclesPerMicrosecond

The actual objects were not created like they were on the T3.x side, so just copied it over from there.

usb.c did not include the usb_rawhid.h so it would give a compiler warning about having to create a ...

Decided to go ahead and move the TX Buffer there as well
Did a quick check in earlier test program to echo the contents back.

The other test program was already setup to receive data on the HID and does hex dump of buffer which looks correct.  So far so good.

Testing against issues of hang from thread:
https://forum.pjrc.com/threads/59881-RawHID-recv()-hangs-after-updating-to-Teensyduino-1-4-9-or-higher-for-Teensy-4-0/page2

F_CPU -> F_CPU_ACTUAL

- trap unused memory regions
- trap nullptr
- trap stack overflow

Fix comment re: systick

fix EWM address

#define FLASHMEM

be compatible with teensy 4