Various fixes and suggestions

[quchen]

Some active reading, some opinions. This is more of a basis for discussion rather than a please-merge-as-is.

Notes:

• I write Vec is the type constructor, not Vec<>. Reason: You would say f: bool -> bool, not f(): bool -> bool.
• I reflowed my input for easier line referencing, but that can be undone before merging.

• fn f(x: !) {} is called not very useful, that’s not true!

  fn ex_falso<T>(x: !) -> T {
    panic!("You proved falsehood exists, the universe will now deconstruct. I hope you’re happy")
  }
  
  fn testi_mc_testface() {
    let always_ok: Result<(), !> = Ok(());
    match always_ok {
        Ok(_) => println!("ok"),
        Err(e) => ex_falso(e), // Commenting this out yields a inexhaustive match
    }
  }

[ralfbiedert]

As discussed, will merge and make some adjustments. Thanks!