Implement __bool__ and __len__ via unary_op virtual method for all types.

__bool__() and __len__() are just the same as __neg__() or __invert__(),
and require efficient dispatching implementation (not requiring search/lookup).
type->unary_op() is just the right choice for this short of adding
standalone virtual method(s) to already big mp_obj_type_t structure.

py/objnone.c

@@ -18,7 +18,7 @@ static void none_print(void (*print)(void *env, const char *fmt, ...), void *env
 
 static mp_obj_t none_unary_op(int op, mp_obj_t o_in) {
     switch (op) {
-        case RT_UNARY_OP_NOT: return mp_const_true;
+        case RT_UNARY_OP_BOOL: return mp_const_false;
         default: return MP_OBJ_NULL; // op not supported for None
     }
 }