Addresses Issue #203.

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

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

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

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.

Still need to make built-ins by these names, and write tests.

Creating of classes (types) and instances is much more like CPython now.
You can use "type('name', (), {...})" to create classes.

It's not really about that, though; it's about me figuring out a sane
way forward for keyword-argument functions (and function
metadata). But it's useful as is, and shouldn't break any existing
code, so here you have it; I'm going to park it in my mind for a bit
while sorting out the rest of the dict branch.

Now much more inline with how CPython does types.

A big change.  Micro Python objects are allocated as individual structs
with the first element being a pointer to the type information (which
is itself an object).  This scheme follows CPython.  Much more flexible,
not necessarily slower, uses same heap memory, and can allocate objects
statically.

Also change name prefix, from py_ to mp_ (mp for Micro Python).