Remove unrenaming from the PIR generators

[effectfully]

QuviQ had a trouble using our PIR type checker with their PIR terms generator: they had to do introduce unrenaming of escaped data type variables, we couldn't figure out a way of avoiding that and so we now have to maintain that disgusting unrenaming in there. Can we do any better? We've had quite a lot of discussion on the Plutus mailing list about the topic (thread for those who have access), but nothing really came out of it. Perhaps we're stuck with this nonsense forever.