Implement modules as singletons Python semantics.

In Python, importing module several times returns same underlying module
object. This also fixes import statement handling for builtin modules.

There're still issues:
1. CPython exposes set of loaded modules as sys.modules, we may want to
do that either.
2. Builtin modules are implicitly imported, which is not really correct.
We should separate registering a (builtin) module and importing a module.
CPython keeps builtin module names in sys.builtin_module_names .

CMSIS V3.01 -> V3.20. Now in stm/cmsis to keep separate from ST
libraries.

Exceptions know source file, line and block name.

Also tidy up some debug printing functions and provide a global
flag to enable/disable them.

Add lean ("raw") socket module.

Add skeleton implementation of array.array and bytearray.

Byte code has a map from byte-code offset to source-code line number,
used to give better error messages.

Now searches both locals_dict and methods.  Partly addresses Issue #145.

So far, only storage, initialization, repr() and buffer protocol is
implemented - alredy suitable for passing binary data around.

Now working for class-defined methods: __getitem__, __setitem__,
__add__, __sub__.  Easy to add others.

Add store_item() virtual method to type to implement container[index] = val

mp_obj_int_get() can be used when just full resolution of C machine_int_t
is required (returns truncated value of long int). mp_obj_int_get_checked()
will throw exception if Python int value not representable in machine_int_t.

Change state layout in VM so the stack starts at state[0] and grows
upwards.  Locals are at the top end of the state and number downwards.
This cleans up a lot of the interface connecting the VM to C: now all
functions that take an array of Micro Python objects are in order (ie no
longer in reverse).

Also clean up C API with keyword arguments (call_n and call_n_kw
replaced with single call method that takes keyword arguments).  And now
make_new takes keyword arguments.

emitnative.c has not yet been changed to comply with the new order of
stack layout.