Fix missing or wrong implementations in node.go

[leafypout]

Some types in ast/node.go were not implementing the Node type, and some implementations were missing Idx0 and Idx1 functions for their type, which results in errors when attempting to use/import some of the types. I have corrected all the faulty implementations. I believe I have implemented the Idx0 and Idx1 functions correctly, but please review those changes and see if I did anything wrong.