py/runtime: Implement dispatch for "reverse op" special methods.

If, for class X, X.__add__(Y) doesn't exist (or returns NotImplemented),
try Y.__radd__(X) instead.

This patch could be simpler, but requires undoing operand swap and
operation switch to get non-confusing error message in case __radd__
doesn't exist.

py/objtype.c

@@ -441,6 +441,13 @@ const qstr mp_binary_op_method_name[] = {
     MP_BINARY_OP_INPLACE_TRUE_DIVIDE,
     MP_BINARY_OP_INPLACE_MODULO,
     MP_BINARY_OP_INPLACE_POWER,*/
+
+    #if MICROPY_PY_REVERSE_SPECIAL_METHODS
+    [MP_BINARY_OP_REVERSE_ADD] = MP_QSTR___radd__,
+    [MP_BINARY_OP_REVERSE_SUBTRACT] = MP_QSTR___rsub__,
+    [MP_BINARY_OP_REVERSE_MULTIPLY] = MP_QSTR___rmul__,
+    #endif
+
     [MP_BINARY_OP_LESS] = MP_QSTR___lt__,
     [MP_BINARY_OP_MORE] = MP_QSTR___gt__,
     [MP_BINARY_OP_EQUAL] = MP_QSTR___eq__,