Originally suggested by @overlookmotel. Currently, all of the methods relating to getting a node's parent ID/kind/node return Option<&AstNode>. However, we know that all AST nodes should have a parent, with the exception of Program. In this case, we could have Program be self-referential: it will return its parent ID as itself, and then we can remove Option from the return type of all methods fetching parent data. This would remove a lot of boilerplate code in the linter and elsewhere. Could potentially improve performance as well by removing lots of unnecessary branches which means fewer instructions.