py: Rename globally-accessible tuple functions, prefix with mp_obj_.

Likely there are other functions that should be renamed, but this is a
start.
diff --git a/py/objnamedtuple.c b/py/objnamedtuple.c
index 0874eea..212cf13 100644
--- a/py/objnamedtuple.c
+++ b/py/objnamedtuple.c
@@ -155,12 +155,12 @@
     o->base.name = name;
     o->base.print = namedtuple_print;
     o->base.make_new = namedtuple_make_new;
-    o->base.unary_op = tuple_unary_op;
-    o->base.binary_op = tuple_binary_op;
+    o->base.unary_op = mp_obj_tuple_unary_op;
+    o->base.binary_op = mp_obj_tuple_binary_op;
     o->base.load_attr = namedtuple_load_attr;
     o->base.store_attr = namedtuple_store_attr;
-    o->base.subscr = tuple_subscr;
-    o->base.getiter = tuple_getiter;
+    o->base.subscr = mp_obj_tuple_subscr;
+    o->base.getiter = mp_obj_tuple_getiter;
     o->base.bases_tuple = (mp_obj_t)&namedtuple_base_tuple;
     o->fields = fields;
     return o;