message: Let builtin prints variables
  After the refactoring of how let and other builtins were parsed, we
  lost the ability to get a list of local variables just by supplying
  the let command without any arguments. This will re-implement that
  ability.
diff: @@ -170,13 +170,20 @@ impl Builtin {
  );
  insert_builtin!("exists", builtin_exists, "Performs tests on files and text");
  insert_builtin!("ion-docs", ion_docs, "Opens the Ion manual");
  -
  + insert_builtin!("let", list_vars, "Displays a list of local variables");
  commands
  }
  }
  // Definitions of simple builtins go here
  +fn list_vars(_: &[&str], shell: &mut Shell) -> i32 {
  + shell.variables.variables.iter().for_each(
  + |(key, val)| println!("{} = {}", key, val)
  + );
  + SUCCESS
  +}
  +
  fn builtin_cd(args: &[&str], shell: &mut Shell) -> i32 {
  match shell.directory_stack.cd(args, &shell.variables) {
  Ok(()) => SUCCESS,
  