py: Implement __reversed__ slot.

Addresses issue #1073.

py/objreversed.c

@@ -39,6 +39,13 @@ typedef struct _mp_obj_reversed_t {
 STATIC mp_obj_t reversed_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
     mp_arg_check_num(n_args, n_kw, 1, 1, false);
 
+    // check if __reversed__ exists, and if so delegate to it
+    mp_obj_t dest[2];
+    mp_load_method_maybe(args[0], MP_QSTR___reversed__, dest);
+    if (dest[0] != MP_OBJ_NULL) {
+        return mp_call_method_n_kw(0, 0, dest);
+    }
+
     mp_obj_reversed_t *o = m_new_obj(mp_obj_reversed_t);
     o->base.type = type_in;
     o->seq = args[0];

py/qstrdefs.h

@@ -74,6 +74,7 @@ Q(__gt__)
 Q(__eq__)
 Q(__le__)
 Q(__ge__)
+Q(__reversed__)
 
 Q(micropython)
 Q(bytecode)

tests/basics/builtin_reversed.py

@@ -31,3 +31,9 @@ def __getitem__(self, pos):
         return pos + 1
 for a in reversed(A()):
     print(a)
+
+# user object with __reversed__
+class B:
+    def __reversed__(self):
+        return [1, 2, 3]
+print(reversed(B()))