Conflicts:
	py/objstr.c
	py/py.mk
	py/stream.c
	unix/main.c
	unix/socket.c

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

Note that splitting by explicit string is not implemented so far.

Also, support negative indexes.

Currently, MicroPython strings are mix between CPython byte and unicode
strings. So, conversion is null so far. This dummy implementation is
intended for compatibility with CPython (so, same code can run on both).

CPython _socket actually have only those and doesn't provide stream
interface (higher-level CPython "socket" what adds this).

+516 bytes x86.

A file cannot be iterated concurrently, so we make io.FileIO its own
iterator.

Uses stream_unbuffered_readline underline.

Useful as getiter method for objects which are their own iterators, etc.

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.