This function computes (x**y)%z in an efficient way.  For large arguments
this operation is otherwise not computable by doing x**y and then %z.

It's currently not used, but is added in case it's useful one day.

For these 3 bitwise operations there are now fast functions for
positive-only arguments, and general functions for arbitrary sign
arguments (the fast functions are the existing implementation).

By default the fast functions are not used (to save space) and instead
the general functions are used for all operations.

Enable MICROPY_OPT_MPZ_BITWISE to use the fast functions for positive
arguments.

Before this patch, the native types for uint and ptr/ptr8/ptr16/ptr32
all overlapped and it was possible to make a mistake in casting.  Now,
these types are all separate and any coding mistakes will be raised
as runtime errors.

Eg: '{:{}}'.format(123, '>20')

@pohmelie was the original author of this patch, but @dpgeorge made
significant changes to reduce code size and improve efficiency.

Fixes use of uninitialized structure field by tinf_uncompress().

Fixes issue #1812.

Previous to this patch the DMA was setup and then the I2C address sent.
If the I2C address sending failed (eg no I2C device on the bus) then the
DMA was left in an inconsistent state.

This patch moves the DMA setup to after a successful sending of the I2C
address(es).

See issue #1765.

USB CDC no longer needs TIM3 (which was originally used for LED(4) PWM)
and so TIM3 has been freed for general purpose use by the user.  Hence
LED(4) lost its PWM capabilities.

This patch reinstates the PWM capabilities using a semi-generic piece
of code which allows to configure a timer and PWM channel to use for any
LED.  But the PWM capability is only configured if the LED is set to an
intensity between 1 and 254 (ie only when needed).  In that case the
relevant timer is configured for PWM.  It's up to the user to make sure
the timers are not used if PWM is active.

This patch also makes sure that PWM LEDs are turned off using standard
GPIO when calling led.off() or led.intensity(0), instead of just setting
the PWM counter to zero.

TIM3 is no longer used by USB CDC for triggering outgoing data, so we
can now make it available to the user.

PWM fading on LED(4) is now gone, but will be reinstated in a new way.

Previous to this patch the USB CDC driver used TIM3 to trigger the
sending of outgoing data over USB serial.  This patch changes the
behaviour so that the USB SOF interrupt is used to trigger the processing
of the sending.  This reduces latency and increases bandwidth of outgoing
data.

Thanks to Martin Fischer, aka @hoihu, for the idea and initial prototype.

See PR #1713.

For single prec, exponents never get larger than about 37.  For double
prec, exponents can be larger than 99 and need 3 bytes to format.  This
patch makes the number of bytes needed configurable.

Addresses issue #1772.

Calling it from mp_init() is too late for some ports (like Unix), and leads
to incomplete stack frame being captured, with following GC issues. So, now
each port should call mp_stack_ctrl_init() on its own, ASAP after startup,
and taking special precautions so it really was called before stack variables
get allocated (because if such variable with a pointer is missed, it may lead
to over-collecting (typical symptom is segfaulting)).

When using newer glibc's the compiler automatically sets
_FORTIFY_SOURCE when building with -O1 and this causes
a special inlined version of printf to be declared which
then bypasses our version of printf.

MP_BC_NOT was removed and the "not" operation made a proper unary
operator, and the opcode format table needs to be updated to reflect
this change (but actually the change is only cosmetic).

Supported return types are: object, bool, int, uint.

For example:

@micropython.asm_thumb
def foo(r0, r1) -> uint:
    add(r0, r0, r1)

Includes full version of pins.csv file with correct mapping of the Nucleo
STM32F401RE chip (LQFP64 package).

Functions added are:
- randint
- randrange
- choice
- random
- uniform

They are enabled with configuration variable
MICROPY_PY_URANDOM_EXTRA_FUNCS, which is disabled by default.  It is
enabled for unix coverage build and stmhal.

SHA1 is used in a number of protocols and algorithm originated 5 years ago
or so, in other words, it's in "wide use", and only newer protocols use
SHA2.

The implementation depends on axTLS enabled. TODO: Make separate config
option specifically for sha1().

micropython.stack_use() returns an integer being the number of bytes used
on the stack.

micropython.heap_lock() and heap_unlock() can be used to prevent the
memory manager from allocating anything on the heap.  Calls to these are
allowed to be nested.

This allows FROZEN_DIR=some-directory to be specified on the make
command line, which will then add all of the files contained within
the indicated frozen directory as frozen files in the image.

There is no change in flash/ram usage if not using the feature.

This is especially useful on smaller MCUs (like the 401) which only
has 64K flash file system.

By reserving enough space for peer address.

Implement enough of statvfs to determine the amount of free
space on a volume.