Do we want to delete Wire exprs in generated egglog commands?

[gussmith23]

Related to #92.

Sometimes Wire expressions don't actually end up unioned with anything. E.g. if we have an unused port on a module connected to a placeholder Wire. We don't necessarily want to delete that Wire, because it is in fact useful!

We can likely detect at egglog codegen time whether we can delete a wire or not. If it's been unioned with something, delete it.