Use System.identityHashCode in Attribute#hashCode

[norswap]

• Attribute#equals uses object identity to compares its fields, so its unecessary (and in fact, potentially counter-productive) to compute the full hashCode of its fields in the hashCode method.
• A student stumbled upon an issue where the hashCode of the Attribute changed because he made a change to the underyling AST. While this is not strictly a bug (AST should be immutable), it would have been avoided by using identityHashCode.
• This will improve performance a lot: the current implem of SighNode#hashCode uses reflection and is very inefficient.