Audit catalog table Datum conversions

A long while back (#408), we found that we were accessing a 64-bit column using 32-bit methods. I'm pretty sure we've added more places we're doing this (nodeid, groupid seem suspicious). Some sort of tooling should be able to catch this (I imagine some sort of assert-only function call against a `TupleDesc`), but for the immediate future it would be good to look over places in our code that access such types and make sure they're always unmarshaled correctly.