Make VM stack grow upwards, and so no reversed args arrays.

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.

py/objfloat.c

@@ -24,8 +24,9 @@ static void float_print(void (*print)(void *env, const char *fmt, ...), void *en
     print(env, "%.8g", o->value);
 }
 
-// args are reverse in the array
-static mp_obj_t float_make_new(mp_obj_t type_in, int n_args, const mp_obj_t *args) {
+static mp_obj_t float_make_new(mp_obj_t type_in, uint n_args, uint n_kw, const mp_obj_t *args) {
+    // TODO check n_kw == 0
+
     switch (n_args) {
         case 0:
             return mp_obj_new_float(0);