Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Expose memory stats functions via "micropython" module.

These are micropython.mem_total(), .mem_current(), .mem_peak(). These are 3
individual functions with simple scalar return value to make sure that
calls to these functions themselves have minimal (hopefully zero) impact on
memory allocation.

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.