This patch eliminates the need for a nested parse node for assignments
and keyword arguments.  It saves a little bit of RAM when parsing.

Also adds tests specifically for testing constant folding.

With this patch parse nodes are allocated sequentially in chunks.  This
reduces fragmentation of the heap and prevents waste at the end of
individually allocated parse nodes.

Saves roughly 20% of RAM during parse stage.

It's relatively small (between 44 and 56 bytes) and helps to reduce heap
pressure and fragmentation during compilation.

Saves a few bytes of code space and eliminates need for rot_two
bytecode (hence saving RAM and execution time, by a tiny bit).

Function annotations are only needed when the native emitter is enabled
and when the current scope is emitted in viper mode.  All other times
the annotations can be skipped completely.

unix-cpy was originally written to get semantic equivalent with CPython
without writing functional tests.  When writing the initial
implementation of uPy it was a long way between lexer and functional
tests, so the half-way test was to make sure that the bytecode was
correct.  The idea was that if the uPy bytecode matched CPython 1-1 then
uPy would be proper Python if the bytecodes acted correctly.  And having
matching bytecode meant that it was less likely to miss some deep
subtlety in the Python semantics that would require an architectural
change later on.

But that is all history and it no longer makes sense to retain the
ability to output CPython bytecode, because:

1. It outputs CPython 3.3 compatible bytecode.  CPython's bytecode
changes from version to version, and seems to have changed quite a bit
in 3.5.  There's no point in changing the bytecode output to match
CPython anymore.

2. uPy and CPy do different optimisations to the bytecode which makes it
harder to match.

3. The bytecode tests are not run.  They were never part of Travis and
are not run locally anymore.

4. The EMIT_CPYTHON option needs a lot of extra source code which adds
heaps of noise, especially in compile.c.

5. Now that there is an extensive test suite (which tests functionality)
there is no need to match the bytecode.  Some very subtle behaviour is
tested with the test suite and passing these tests is a much better
way to stay Python-language compliant, rather than trying to match
CPy bytecode.

Previous to this patch there were some cases where line numbers for
errors were 0 (unknown).  Now the compiler attempts to give a better
line number where possible, in some cases giving the line number of the
closest statement, and other cases the line number of the inner-most
scope of the error (eg the line number of the start of the function).
This helps to give good (and sometimes exact) line numbers for
ViperTypeError exceptions.

This patch also makes sure that the first compile error (eg SyntaxError)
that is encountered is reported (previously it was the last one that was
reported).

ViperTypeError now includes filename and function name where the error
occurred.  The line number is the line number of the start of the
function definition, which is the best that can be done without a lot
more work.

Partially addresses issue #1381.

Previous to this patch each time a bytes object was referenced a new
instance (with the same data) was created.  With this patch a single
bytes object is created in the compiler and is loaded directly at execute
time as a true constant (similar to loading bignum and float objects).
This saves on allocating RAM and means that bytes objects can now be
used when the memory manager is locked (eg in interrupts).

The MP_BC_LOAD_CONST_BYTES bytecode was removed as part of this.

Generated bytecode is slightly larger due to storing a pointer to the
bytes object instead of the qstr identifier.

Code size is reduced by about 60 bytes on Thumb2 architectures.

This fixes a long standing problem that viper code generation gave
terrible error messages, and actually no errors on pyboard where
assertions are disabled.

Now all compile-time errors are raised as proper Python exceptions, and
are of type ViperTypeError.

Addresses issue #940.

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.

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

These allow to fine-tune the compiler to select whether it optimises
tuple assignments of the form a, b = c, d and a, b, c = d, e, f.
Sensible defaults are provided.

Just to reduce code size.  Messages are still to the point and
unambiguous.

Addresses issue #1140.

Saves around 60 bytes code on Thumb2 archs.

Saves 116 bytes for stmhal and 56 bytes for cc3200 port.

Reduces code size by 72 bytes on Thumb2 archs.

Saves about 250 code bytes for Thumb2 archs.

Also gives line number of location of error.  Very useful!

Previous to this patch, a big-int, float or imag constant was interned
(made into a qstr) and then parsed at runtime to create an object each
time it was needed.  This is wasteful in RAM and not efficient.  Now,
these constants are parsed straight away in the parser and turned into
objects.  This allows constants with large numbers of digits (so
addresses issue #1103) and takes us a step closer to #722.

To enable parsing constants more efficiently, mp_parse should be allowed
to raise an exception, and mp_compile can already raise a MemoryError.
So these functions need to be protected by an nlr push/pop block.

This patch adds that feature in all places.  This allows to simplify how
mp_parse and mp_compile are called: they now raise an exception if they
have an error and so explicit checking is not needed anymore.

This cleans up vstr so that it's a pure "variable buffer", and the user
can decide whether they need to add a terminating null byte.  In most
places where vstr is used, the vstr did not need to be null terminated
and so this patch saves code size, a tiny bit of RAM, and makes vstr
usage more efficient.  When null termination is needed it must be
done explicitly using vstr_null_terminate.