Allow the `part_1` and `part_2` functions to have a generic argument

[giacomocavalieri]

When experimenting with parsing it's really useful to just debug print it like this:

fn part_1(input) {
  io.debug(input)
}

However, gladvent complains that input has a generic type. I think it would be really nice if we could get rid of this limitation

[TanklesXL]

good idea