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 .
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.
