message: use slice instead of vec argument
diff: @@ -28,7 +28,7 @@ pub trait Pretty {
  fn to_pretty(&self) -> Val;
  }
  -fn longest_key_length(ast: &AST) -> usize {
  +fn longest_key_length(ast: &[(Key, Val)]) -> usize {
  ast.iter()
  .fold(0, |longest, (key, _)| std::cmp::max(longest, key.len()))
  }
  