Support symmetric_values and symmetric role

[jp-fizzbee]

Symmetry reduction by specifying values and roles as symmetric.

For ids and keys, you can specify
```
  KEYS = symmetric_values('key', 3)
  VALUES = symmetric_values('val', 3)
```

For roles,

```
symmetric role Node:
  # actions and functions
  pass

```

To access the id of the node to be used at other places, use `role.__id__`