Add support for complimentary channel output and deadtime.

https://github.com/pfalcon/re1.5

https://github.com/pfalcon/re1.5

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.

unix: Rename "microsocket" module to "usocket".

Per new conventions, we'd like to consistently use "u*" naming conventions
for modules which don't offer complete CPython compatibility, while offer
subset or similar API.

Updating README.md. The Unix build requires pkg-config for FFI.

Updating README.md to include the fact that the Unix build requires pkg-config to build the FFI module.

Specifically, at least Ubuntu's i586-mingw32msvc-gcc doesn't supply
__LITTLE_ENDIAN__ and friends. And as it's safe enough to assume that
Windows is only little-endian, then it's defined unconditionally,
instead of duplicating detection logic in py/mpconfig.h (or adding
windows-specific defines to it).

Should address issue #856.

Partly addresses issue #856.

If micropython.native decorator doesn't compile, then we skill all
native/viper tests.

This patch also re-enables the ujson_loads test on NT.

Addresses issue #861, and partially addresses issue #856.

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.

This has benefits all round: code factoring for parse/compile/execute,
proper context save/restore for exec, allow to sepcify globals/locals
for eval, and reduced ROM usage by >100 bytes on stmhal and unix.

Also, the call to mp_parse_compile_execute is tail call optimised for
the import code, so it doesn't increase stack memory usage.