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.

This allows a user-specified Timer for the triggering of the ADC read,
mirroring the new behaviour of DAC.write_timed.

Addresses issue #1129.

This allows the DAC to use a user-specified Timer for the triggering
(instead of the default Timer(6)), while still supporting original
behaviour.

Addresses issues #1129 and #1388.

Because if the counter is above the new value of the auto-reload register
then it may be a long time until the timer wraps around.

Saves 68 bytes.

Previous to this patch the printing mechanism was a bit of a tangled
mess.  This patch attempts to consolidate printing into one interface.

All (non-debug) printing now uses the mp_print* family of functions,
mainly mp_printf.  All these functions take an mp_print_t structure as
their first argument, and this structure defines the printing backend
through the "print_strn" function of said structure.

Printing from the uPy core can reach the platform-defined print code via
two paths: either through mp_sys_stdout_obj (defined pert port) in
conjunction with mp_stream_write; or through the mp_plat_print structure
which uses the MP_PLAT_PRINT_STRN macro to define how string are printed
on the platform.  The former is only used when MICROPY_PY_IO is defined.

With this new scheme printing is generally more efficient (less layers
to go through, less arguments to pass), and, given an mp_print_t*
structure, one can call mp_print_str for efficiency instead of
mp_printf("%s", ...).  Code size is also reduced by around 200 bytes on
Thumb2 archs.

This also cleans up spurious interrupts which happen at timer
initilaization time.

In particular, make sure that the globals are all initialized
before enabling the interrupt, and also make sure that the timer
interrupt has been initialied before enabling the NVIC.

Addresses issue #1113.

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

The function is modeled after traceback.print_exception(), but unbloated,
and put into existing module to save overhead on adding another module.
Compliant traceback.print_exception() is intended to be implemented in
micropython-lib in terms of sys.print_exception().

This change required refactoring mp_obj_print_exception() to take pfenv_t
interface arguments.

Addresses #751.

This patch enables output on the complimentary channels (TIMx_CHyN).
For timers 1 and 8, deadtime can also be inserted when the channels
transition. For the pyboard, TIM8_CH1/CH1N and TIM8_CH2/CH2N can
take advantage of this.

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.

Timers now have the following new features:
- can init freq using floating point; eg tim.init(freq=0.1)
- tim.source_freq() added to get freq of timer clock source
- tim.freq() added to get/set freq
- print(tim) now prints freq

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.

Also improve precision of calculating PWM percent in integer mode.
Also update teensy with edge case fix.

Teensy doesn't need to worry about overflows since all of
its timers are only 16-bit.

For PWM, the pulse width needs to be able to vary from 0..period+1
(pulse-width == period+1 corresponds to 100% PWM)

I couldn't test the 0xffffffff cases since we can't currently get a
period that big in python. With a prescaler of 0, that corresponds
to a freq of 0.039 (i.e. cycle every 25.56 seconds), and we can't
set that using freq or period.

I also tested both stmhal and teensy with floats disabled, which
required a few other code changes to compile.

Also do the same for teensy timer code.

Fix stmhal and teensy print routines to report actual prescaler an period.
Fix teensy build to use soft-float
Add USE_ARDUINO_TOOLCHAIN option to teensy build

This allows to set the pulse width (for PWM mode) as a ratio relative to
the period of the timer.  Eg, 0.5 is a 50% duty cycle.  You can set the
ratio in the channel init, or using channel.pulse_width_ratio; the
latter can also read the pulse width as a ratio.

Addressing issue #50, still some way to go yet.

See discussion in issue #50.

This fixes #733.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Make include dependencies neater, and adheres to the coding convention
that headers should not include headers.