Usage:

    import machine
    wdt = machine.WDT(0, 5000) # 5 second timeout
    wdt.feed()

Thanks to Moritz for the initial implementation.

Also raise an exception for machine.freq and machine.deepsleep on this
MCU, since they are not yet implemented.

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.

They are no longer needed because stm constants can now be 32 bits wide.

This uses the newly factored machine_mem functions.

To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").

Fixes #1701.

If RTC is already running at boot then it's left alone.  Otherwise, RTC is
started at boot but startup function returns straight away.  RTC startup
is then finished the first time it is used.  Fallback to LSI if LSE fails
to start in a certain time.

Also included:
 MICROPY_HW_CLK_LAST_FREQ
        hold pyb.freq() parameters in RTC backup reg
 MICROPY_HW_RTC_USE_US
        option to present datetime sub-seconds in microseconds
 MICROPY_HW_RTC_USE_CALOUT
        option to enable RTC calibration output

CLK_LAST_FREQ and RTC_USE_CALOUT are enabled for PYBv1.0.

In new hardware API, these classes implement master modes of interfaces,
and "mode" parameter is not accepted. Trying to implement new HW API
in terms of older pyb module leaves variuos corner cases:

In new HW API, I2C(1) means "I2C #1 in master mode" (? depends on
interpretation), while in old API, it means "I2C #1, with no settings
changes".

For I2C class, it's easy to make mode optional, because that's last
positional param, but for SPI, there's "baudrate" after it (which
is inconsistent with I2C, which requires "baudrate" to be kwonly-arg).

py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for.  A
port will also provide mphalport.h with further HAL declarations.

Looks like we can use the same Pin class for legacy pyb module and new
machine module.

This can get PYBv1.0 stop current down to around 290uA.

This helps make files reusable across other ports.

Now all ports can use pybstdio.c to provide sys.stdin/stdout/stderr, so
long as they implement mp_hal_stdin_* and mp_hal_stdout_* functions.

Different HID modes can be configured in Python.  You can either use
predefined mouse or keyboard, or write your own report descriptor.

A GC in stmhal port now only scans true root pointers, not entire BSS.
This reduces base GC time from 1700ms to 900ms.

This is useful if you need precise control over the speed of
peripherals (eg SPI clock).

pyb.delay and pyb.udelay now use systick if IRQs are enabled, otherwise
they use a busy loop.  Thus they work correctly when IRQs are disabled.
The busy loop is computed from the current CPU frequency, so works no
matter the CPU frequency.

This is just a clean-up of the code.  Generated code is exactly the
same.

Also restrict higher frequencies to have a VCO_OUT frequency below
432MHz, as specified in the datasheet.

Docs improved to list allowed frequencies, and explain about USB
stability.

Found these by compiling stmhal with mp_uint_t of type uint32_t instead
of unsigned int.  This actually makes a difference to the code, but just
a curiosity.

Eg pyb.freq(120000000) sets the CPU to 120MHz.  The frequency can be set
at any point in the code, and can be changed as many times as you like.
Note that any active timers will need to be reconfigured after a freq
change.

Valid range is 24MHz to 168MHz (but not all freqs are supported).  The
code maintains a 48MHz clock for the USB at all times and it's possible
to change the frequency at a USB REPL and keep the REPL alive (well,
most of the time it stays, sometimes it resets the USB for some reason).
Note that USB does not work with pyb.freq of 24MHz.