Make mp_obj_str_get_data return char* instead of byte*.

Can't decide which is better for string type, char or byte pointer.
Changing to char removes a few casts.  Really need to do proper unicode.
diff --git a/py/objarray.c b/py/objarray.c
index c595d21..75ed399 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -174,7 +174,7 @@
     }
     // TODO check args
     uint l;
-    const byte *typecode = mp_obj_str_get_data(args[0], &l);
+    const char *typecode = mp_obj_str_get_data(args[0], &l);
     if (n_args == 1) {
         return array_new(*typecode, 0);
     }