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/objtuple.c b/py/objtuple.c
index a64b1fa..fd6d415 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -31,8 +31,9 @@
     print(env, ")");
 }
 
-// args are in reverse order in the array
-static mp_obj_t tuple_make_new(mp_obj_t type_in, int n_args, const mp_obj_t *args) {
+static mp_obj_t tuple_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 empty tuple
@@ -119,19 +120,6 @@
     return o;
 }
 
-mp_obj_t mp_obj_new_tuple_reverse(uint n, const mp_obj_t *items) {
-    if (n == 0) {
-        return mp_const_empty_tuple;
-    }
-    mp_obj_tuple_t *o = m_new_obj_var(mp_obj_tuple_t, mp_obj_t, n);
-    o->base.type = &tuple_type;
-    o->len = n;
-    for (int i = 0; i < n; i++) {
-        o->items[i] = items[n - i - 1];
-    }
-    return o;
-}
-
 void mp_obj_tuple_get(mp_obj_t self_in, uint *len, mp_obj_t **items) {
     assert(MP_OBJ_IS_TYPE(self_in, &tuple_type));
     mp_obj_tuple_t *self = self_in;