Per the previously discussed plan. mount() still stays backward-compatible,
and new mkfs() is rought and takes more args than needed. But is a step
in a forward direction.

This allows you to pass a number (being an address) to a viper function
that expects a pointer, and also allows casting of integers to pointers
within viper functions.

This was actually the original behaviour, but it regressed due to native
type identifiers being promoted to 4 bits in width.

If MICROPY_VFS_FAT is defined, mp_type_fileio & mp_type_textio won't be
defined, as these may be alredy defined elsewhere. The idea is to have
compartmentalized VFS FatFs class, which can work in parallel with some
other "main" filesystem. E.g., for unix port, mp_type_fileio, etc. will
be defined for the main POSIX filesystem, while stmhal/file.c will be
a self-contained VFS file class.

Move definition of mp_builtin_open_obj to a separate module, then file.c
becomes more or less compartmentalized FatFs file class, which can be used
together with file class implementations for other (V)FSes.

Don't repeat MP_STATE_PORT(fs_user_mount), use local var.

That's not true e.g. on Linux.

To allow to reuse stmhal/diskio for ports which don't have flash but have
other storage devices.

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)