hivex: Add value_any callback to the visitor.

hivex: Add value_any callback to the visitor.

The visitor currently contains lots of value_* callbacks, such as
value_string which is called back when the value has type string.

This is fine but it makes it complicated to deal with the case where
you just want to see 'a value', and don't care about its type.

The value_any callback allows visitors to see values generically.