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.

Without mp_sys_path and mp_sys_argv in the root pointer section of the
state, their memory was being incorrectly collected by GC.

Also fix list.sort so it works with user-defined types, and parse the
keyword arguments properly.

Addresses issue #338.

Only noticeable difference is how newlines are encoded in triple-quoted
strings.  The behaviour now matches CPython3.

Addresses issue #1014.

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.

- This then provides support for floats in the struct package

Eg, "() + 1" now tells you that __add__ is not supported for tuple and
int types (before it just said the generic "binary operator").  We reuse
the table of names for slot lookup because it would be a waste of code
space to store the pretty name for each operator.

Defining NDEBUG (to any value, even 0) disables debugging.  Otherwise,
if it's not defined, debugging is enabled.

This makes the code (more) compatible with the C99 standard.

- namedtuple was wrongly using MP_OBJ_QSTR_VALUE instead of mp_obj_str_get_qstr,
so when passed a non-interned string it would segfault; fix this by using mp_obj_str_get_qstr
- store the namedtuple field names as qstrs so it is not needed to use mp_obj_str_get_qstr
everytime the field name has to be accessed. This also slighty increases performance when
fetching attributes

After vstr refactor. Fixes #1084.

There was really weird warning (promoted to error) when building Windows
port. Exact cause is still unknown, but it uncovered another issue:
8-bit and unicode str_make_new implementations should be mutually exclusive,
and not built at the same time. What we had is that bytes_decode() pulled
8-bit str_make_new() even for unicode build.

To somewhat unbreak -DSTATIC="" compile.

See issue #699.

It needs to be scanned by GC.  Thanks to Daniel Campora.

With this patch str/bytes construction is streamlined.  Always use a
vstr to build a str/bytes object.  If the size is known beforehand then
use vstr_init_len to allocate only required memory.  Otherwise use
vstr_init and the vstr will grow as needed.  Then use
mp_obj_new_str_from_vstr to create a str/bytes object using the vstr
memory.

Saves code ROM: 68 bytes on stmhal, 108 bytes on bare-arm, and 336 bytes
on unix x64.

This patch allows to reuse vstr memory when creating str/bytes object.
This improves memory usage.

Also saves code ROM: 128 bytes on stmhal, 92 bytes on bare-arm, and 88
bytes on unix x64.

This allows an exception to propagate correctly through a finally
handler.

Addresses issue #1073.

Addresses issue #1067.