Reference MCU is dsPIC33J256GP506 with 256k ROM and 8k RAM, on the dsPIC
DSC Starter Kit board.  The REPL works, GC works, pyb module has LED and
Switch objects.  It passes some tests from the test suite (most it can't
run because it doesn't have the Python features enabled).

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.

Current version has been numbered as 0.9.0 since Timers/PWM support
is still missing.

Error reporting is also changed from detailed to terse, as with the
rest of the CC3200's modules. All this combined saves ~200 bytes.

Conditional on MICROPY_PY_ALL_SPECIAL_METHODS.

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

Add example: using named function for the Timer's callback.
Add note: improving traceback inside interrupt timers.

There are lots of cosmetic changes, but this release brings a  very
important bug fix:
 - Fixed f_unlink() does not remove cluster chain of the file.

With R0.10c if you try to write a file that is too large to fit in the
free space of the drive, the operation fails, you delete the incomplete
file, and it seems to be erased, but the space is not really freed,
because any subsequent write operations fail because the drive is
"still" full. The only way to recover from this is by formatting the
drive. I can confirm that R0.11 fixes the problem.

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.