Addresses issue #1285.

It's configurable and only enabled for stmhal port.

That layer is nice, but the CC3200 doesn't need it and getting rid of
it saves ~200 bytes, which are more than welcome.

This is needed to avoid half-open connections.

The reason to have our owm ubinascii module is so that later we
can add crc32 support using the hardware engine.

In particular, dates prior to Mar 1, 2000 are screwed up.

The easiest way to see this is to do:

>>> import time
>>> time.localtime(0)
(2000, 1, 1, 0, 0, 0, 5, 1)
>>> time.localtime(1)
(2000, 1, 2, 233, 197, 197, 6, 2)

With this patch, we instead get:
>>> import time
>>> time.localtime(1)
(2000, 1, 1, 0, 0, 1, 5, 1)

Doh - In C % is NOT a modulo operator, it's a remainder operator.

Effect can be easily achieved by ubinsacii.hexlify(hash.digest()).

This drops the size of unicode_isxdigit from 0x1e + 0x02 filler to
0x14 bytes (so net code reduction of 12 bytes) and will make
unicode_is_xdigit perform slightly faster.

This also pulls out hex_digit from py/lexer.c and makes unichar_hex_digit

This allows using (almost) the same code for printing floats everywhere,
removes the dependency on sprintf and uses just snprintf and
applies an msvc-specific fix for snprintf in a single place so
nan/inf are now printed correctly.

Fixes sign-compare warning.

ISO C forbids conversion between function pointers and void*, gcc
-pedantic triggers a warning.

Background: trying to make an amalgamation of all the code gave some
errors with redefined types and inconsistent use of static.

Can be enabled by defining MICROPY_PORT_WLAN_URN=1 in mpconfigport.h.

When entering the interrupt handler of a given GPIO port, more than
one pin could have pending interrupts, therefore care must be taken
to service each interrupt one by one before leaving.

This reverts commit 8fbabab1. Turned to
cause problems on MacOSX.