Should address issue #475.

py: don't look for any additional headers when lexerunix is disabled

py: the entire `<unistd.h>` shouldn't be needed

Convert sys module to static allocation

objlist: Make .extend accept arbitrary iterable.

Must use mp_obj_get_type to get the type of an object.  Can't assume
mp_obj_t is castable to mp_obj_base_t.

Should fix issue #463.

Pass a single parameter (doesn't matter what): pyb.info(1), will dump
the GC alloc table.

Attempt to address issue #386.  unique_code_id's have been removed and
replaced with a pointer to the "raw code" information.  This pointer is
stored in the actual byte code (aligned, so the GC can trace it), so
that raw code (ie byte code, native code and inline assembler) is kept
only for as long as it is needed.  In memory it's now like a tree: the
outer module's byte code points directly to its children's raw code.  So
when the outer code gets freed, if there are no remaining functions that
need the raw code, then the children's code gets freed as well.

This is pretty much like CPython does it, except that CPython stores
indexes in the byte code rather than machine pointers.  These indices
index the per-function constant table in order to find the relevant
code.

We're not going to implement all the plethora of types in there in C.
Funnily, CPython implements defaultdict in C, and namedtuple in Python.

Similar to similar support for lists.

These largely duplicate str() & bytes() constructors' functionality,
but can be used to achieve Python2 compatibility.

unix: Enable modffi by default.

Improved the Thumb assembler back end.  Added many more Thumb
instructions to the inline assembler.  Improved parsing of assembler
instructions and arguments.  Assembler functions can now be passed the
address of any object that supports the buffer protocol (to get the
address of the buffer).  Added an example of how to sum numbers from
an array in assembler.

ffi is needed to use micropython-lib, so let's have it enabled by default,
then folks who have troubles with libffi can disable it, instead of everyone
doing manual actions again and again.