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.
diff --git a/py/objset.c b/py/objset.c
index 6ea6d62..f44c5ba 100644
--- a/py/objset.c
+++ b/py/objset.c
@@ -46,7 +46,9 @@
 }
 
 
-static mp_obj_t set_make_new(mp_obj_t type_in, int n_args, const mp_obj_t *args) {
+static mp_obj_t set_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 a new, empty set
@@ -239,7 +241,7 @@
     if (MP_OBJ_IS_TYPE(self_in, &set_type)) {
         self = self_in;
     } else {
-        self = set_make_new(NULL, 1, &self_in);
+        self = set_make_new((mp_obj_t)&set_type, 1, 0, &self_in);
         cleanup_self = true;
     }
 
@@ -248,7 +250,7 @@
     if (MP_OBJ_IS_TYPE(other_in, &set_type)) {
         other = other_in;
     } else {
-        other = set_make_new(NULL, 1, &other_in);
+        other = set_make_new((mp_obj_t)&set_type, 1, 0, &other_in);
         cleanup_other = true;
     }
     bool out = true;