Most of the time you don't use the NSS pin of the SPI bus, and so it
shouldn't be enabled by default (this gave some bugs in the past).

In CPython IOError (and EnvironmentError) is deprecated and aliased to
OSError.  All modules that used to raise IOError now raise OSError (or a
derived exception).

In Micro Python we never used IOError (except 1 place, incorrectly) and
so don't need to keep it.

See http://legacy.python.org/dev/peps/pep-3151/ for background.

Fixes bug with x86-64 viper ptr16.

Viper can now do the following:

def store(p:ptr8, c:int):
    p[0] = c

This does a store of c to the memory pointed to by p using a machine
instructions inline in the code.

This included a bit of restructuring of the assembler backends.  Note
that the ARM backend is missing a few functions and won't compile.

Allows things like: if 1: ...

We save some code bytes by using builtin string functions.

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

Added pyb.elapsed_millis and pyb.elapsed_micros

teensy: Enable 8-byte stack alignment for IRQ Handlers.

Fix error: unknown type name 'size_t'

Fix timer overflow code.

tested using:
stmhal: https://github.com/dhylands/upy-examples/blob/master/elapsed.py
teensy: https://github.com/dhylands/upy-examples/blob/master/teensy/elapsed.py

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.

It seems that newer versions of clang don't like the .bss directive, so
we don't use it for OSX.

Addressing issues #865 and #875.

Pulled in and modified work done by mux/iabdalkader on cc3k driver, from
iabdalkader-cc3k-update branch.  That branch was terribly messy and had
too many conflicts to merge neatly.

Images are currently served from micropython.org/static.  I don't know
if there is a better way to handle images.

The JSON specs are relatively flexible and allow us to use one function
to print strings, be they ascii, bytes or utf-8 encoded.

Addresses issue #864.

It seems most sensible to use size_t for measuring "number of bytes" in
malloc and vstr functions (since that's what size_t is for).  We don't
use mp_uint_t because malloc and vstr are not Micro Python specific.

Also do the same for teensy timer code.

Add pulse_width_percent to teensy.