Makes pyexec.c more re-usable for other ports.

- 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.

Actually manage size of the output buffer.

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

So far implements .scan(lambda x: print(x)) function to scan for WiFi access
points.

It uses RAM and on pyboard we are generally tight on RAM, so disable
this optimisation for general builds.  If users need the speed then
they can build their own version.  Maybe in the future we can have
different versions of pyboard firmware built with different tradeoffs.

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

- Tests vary based on build configuration (32/64-bit and internal int type).
- Added tests for exceptions raised on overflow of int type.

- 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

accept might raise an exception, in which case the new socket is not
fully created.  It has a finaliser so will run close() method when GC'd.
Before this patch close would try to close an invalid socket.  Now
fixed.

setsockopt took address of stack value which became out of scope.  Now
fixed.

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.

Addresses issue #1081.

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