A few tests still fail on PYBLITE, and that's due to differences in the
available peripheral block numbers on the different MCUs (eg I2C(2)
exists on one, but it's I2C(3) on the other).

This new function controls what happens on a hard-fault:
- debugging disabled: board will do a reset
- debugging enabled: board will print registers and stack and flash LEDs

The default is disabled, ie to do a reset.  This is different to previous
behaviour which flashed the LEDs and waited indefinitely.

machine.time_pulse_us() is intended to provide very fine timing, including
while working with signal bursts, where each transition is tracked in row.
Throwing and handling an exception may take too much time and "signal loss".
So instead, in case of a timeout, just return negative value. Cases of
timeout while waiting for initial signal stabilization, and during actual
timing, are recognized.

The documentation is updated accordingly, and rewritten somewhat to clarify
the function behavior.

Updated modbuiltin.c to add conditional support for 3-arg calls to
pow() using MICROPY_PY_BUILTINS_POW3 config parameter. Added support in
objint_mpz.c for for optimised implementation.

The vfs_fat_fsusermount test is no longer relevant so has been removed.

The new version of FatFs requires a minimum of 50 blocks on the device.
Also, some tests no longer make sense with an OO vfs.

Starts with concatenation with an empty string.

Fixes #2806.

For a small number of bytes, it's expected to return a small int without
allocation.

This test works only for MICROPY_LONGINT_IMPL == MICROPY_LONGINT_IMPL_MPZ
and needs a way of skipping in other cases.

Previouly, we had errors checked in callers, which led to duplicate code
or missing checks in some places.