Rename rt_* to mp_*.

Mostly just a global search and replace.  Except rt_is_true which
becomes mp_obj_is_true.

Still would like to tidy up some of the names, but this will do for now.
diff --git a/py/objenumerate.c b/py/objenumerate.c
index aaeb254..23a3f70 100644
--- a/py/objenumerate.c
+++ b/py/objenumerate.c
@@ -21,7 +21,7 @@
     assert(n_args > 0);
     mp_obj_enumerate_t *o = m_new_obj(mp_obj_enumerate_t);
     o->base.type = &mp_type_enumerate;
-    o->iter = rt_getiter(args[0]);
+    o->iter = mp_getiter(args[0]);
     o->cur = n_args > 1 ? mp_obj_get_int(args[1]) : 0;
     return o;
 }
@@ -37,7 +37,7 @@
 STATIC mp_obj_t enumerate_iternext(mp_obj_t self_in) {
     assert(MP_OBJ_IS_TYPE(self_in, &mp_type_enumerate));
     mp_obj_enumerate_t *self = self_in;
-    mp_obj_t next = rt_iternext(self->iter);
+    mp_obj_t next = mp_iternext(self->iter);
     if (next == MP_OBJ_NULL) {
         return MP_OBJ_NULL;
     } else {