Implement a substitute for Variable::create_val_repr()

[rameloni]

The create_val_repr() is now used to render a raw_value of a variable properly but a better version should be implemented.

https://github.com/rameloni/tywaves-rs/blame/b1ef8c5259ceb9ff91aeb157bf13cb32ff20f42e/src/tyvcd/spec.rs#L250-L295

The method now does not use the TraceValue of a variable but instead only the input raw_value.