Support `where` clauses
Currently, if someone has a where clause in their module, it's turned into a let binding.
This is because the compiler desugars where clauses during parsing.
This issue is blocked on the compiler supporting where clauses that aren't desugared in parsing because we should not maintain a different parser from the official one if we can at all help it. We can't do anything about it, but we should still track that it's a thing we want to support.