deduplicate infer var instantiation Having 3 separate implementations of one of the most subtle parts of our type system is not a good strategy if we want to maintain a sound type system :sparkles: while working on this I already found some subtle bugs in the existing code, so that's awesome :tada: cc #121159 This was necessary as I am not confident in my nll changes in #119106, so I am first cleaning this up in a separate PR. r? `@BoxyUwU`