Also rename call_args_t to mp_call_args_t.

I.e. in this mode, C stack will never be used to call a Python function,
but if there's no free heap for a call, it will be reported as
RuntimeError (as expected), not MemoryError.

Allow for reuse for stackless design, where preparing args is separate from
calling.

Conditional on MICROPY_PY_ALL_SPECIAL_METHODS.

Per https://docs.python.org/3/library/functions.html#round, 2-args format
guaranteedly returns float.

Disabled by default.  Enabled on unix and windows ports.

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.

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

Saves around 30 bytes code on Thumb2 archs.

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).

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.

Disabled by default.  Enabled on unix and stmhal ports.

"Base" should rather refer to "base type"."Base object for attribute
lookup" should rather be just "object".

Also, a case of common subexpression elimination.

Addresses issue #1160.

Given that there's already support for "fixed table" maps, which are
essentially ordered maps, the implementation of OrderedDict just extends
"fixed table" maps by adding an "is ordered" flag and add/remove
operations, and reuses 95% of objdict code, just making methods tolerant
to both dict and OrderedDict.

Some things are missing so far, like CPython-compatible repr and comparison.

OrderedDict is Disabled by default; enabled on unix and stmhal ports.

We already have branch for lookup->is_type == true, so here it's guaranteed
to be false.