Add a builtin for panic/crash

[Janiczek]

Opening this issue based on Discord discussion.

I'd like to have a way to crash at runtime. For example, Elm programs have Debug.todo which is sometimes useful.

I've tried to use the "cloning non-affine functions" method, but it doesn't work when you're using currying:

type Unit = Unit

panic _ =
  let nonaffine = @a (a,a)
  let boom = (nonaffine,nonaffine)
  (panic (Unit/Unit)) # doesn't work when we use `panic a = let ...; (panic a)`

# successfully crashes (ERROR: attempt to clone a non-affine global reference.)
Basics/append a b = (panic (Unit/Unit))

# doesn't work, hangs CPU
Basics/append = (panic (Unit/Unit))

main = (Basics/append "a" "b")