Disabled by default.  Enabled on unix and windows ports.

Setting the word count before a SPI transaction is only needed when
using DMA.

Increasing it from 1K to 2K gives more freedom to the callback
handlers, before this, simply nesting a function call into a
printf would cause a stack overflow.

This is a temporary fix.

When just the bytecode emitter is needed there is no need to have a
dynamic method table for the emitter back-end, and we can instead
directly call the mp_emit_bc_XXX functions.  This gives a significant
reduction in code size and a very slight performance boost for the
compiler.

This patch saves 1160 bytes code on Thumb2 and 972 bytes on x86, when
native emitters are disabled.

Overall savings in code over the last 3 commits are:

bare-arm: 1664 bytes.
minimal:  2136 bytes.
stmhal:    584 bytes (it has native emitter enabled).
cc3200:   1736 bytes.

First pass for the compiler is computing the scope (eg if an identifier
is local or not) and originally had an entire table of methods dedicated
to this, most of which did nothing.  With changes from previous commit,
this set of methods can be removed and the methods from the bytecode
emitter used instead, with very little modification -- this is what is
done in this commit.

This factoring has little to no impact on the speed of the compiler
(tested by compiling 3763 Python scripts and timing it).

This factoring reduces code size by about 270-300 bytes on Thumb2 archs,
and 400 bytes on x86.

Saves around 230 bytes on Thumb2 and 750 bytes on x86.

Continuation of refactoring applied previously to objlist.

This is intended to improve coverage of the test suite.

It allowed such things as (a, b) += c.

Saves around 30 bytes code on Thumb2 archs.

This allows to properly initialize the system led and add it
to the sleep module so that it can be restored when resuming
from suspended mode.

Unfortunately, these timeouts are the only realiable way (for now), to
be able to detect broken connections due to half-open sockets. Such a
thing occurs when getting out of the WiFi coverage area or when
disconnecting from the AP (sometimes the client doesn't send the
disconnect packet).

mp_obj_t internal representation doesn't have to be a pointer to object,
it can be anything.

There's also a support for back-conversion in the form of MP_OBJ_UNCAST.
This is kind of optimization/status quo preserver to minimize patching the
existing code and avoid doing potentially expensive MP_OBJ_CAST over and
over. But then one may imagine implementations where MP_OBJ_UNCAST is very
expensive. But such implementations are unlikely interesting in practice.

This change is required to unbreak some CPython stdlib modules (as included
into micropython-lib).

The 2 removed from coverage build are: -Wredundant-decls and
-Wstrict-prototypes.

To have proper low power mode, need to configure all unused pins in
input mode, so need to have them available.

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

Despite initial guess, this code factoring does not hamper performance.
In fact it seems to improve speed by a little: running pystone(1.2) on
pyboard (which gives a very stable result) this patch takes pystones
from 1729.51 up to 1742.16.  Also, pystones on x64 increase by around
the same proportion (but it's much noisier).

Taking a look at the generated machine code, stack usage with this patch
is unchanged, and call is tail-optimised with all arguments in
registers.  Code size decreases by about 50 bytes on Thumb2 archs.

Only MASTER mode is supported. Transfer width is configurable to
8, 16 or 32 bits.