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/showbc.c b/py/showbc.c
index 2ba81df..36393ce 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -286,12 +286,11 @@
                 printf("BUILD_MAP " UINT_FMT, unum);
                 break;
 
-                /*
             case MP_BC_STORE_MAP:
-                sp += 2;
-                rt_store_map(sp[0], sp[-2], sp[-1]);
+                printf("STORE_MAP");
                 break;
 
+                /*
             case MP_BC_MAP_ADD:
                 DECODE_UINT;
                 // I think it's guaranteed by the compiler that sp[unum + 1] is a map
@@ -310,6 +309,13 @@
                 printf("SET_ADD " UINT_FMT, unum);
                 break;
 
+#if MICROPY_ENABLE_SLICE
+            case MP_BC_BUILD_SLICE:
+                DECODE_UINT;
+                printf("BUILD_SLICE " UINT_FMT, unum);
+                break;
+#endif
+
             case MP_BC_UNPACK_SEQUENCE:
                 DECODE_UINT;
                 printf("UNPACK_SEQUENCE " UINT_FMT, unum);