Adds a lot of code, makes IRQs a bit less efficient, but is very useful
for debugging.  Usage: pyb.irq_stats() returns a memory view that can be
read and written, eg:

    list(pyb.irq_stats())
    pyb.irq_stats()[0]
    pyb.irq_stats()[0] = 0

The patch provides general IRQ_ENTER() and IRQ_EXIT() macros that can be
modified to provide further IRQ statistics if desired.