Prism wall strength should be pattern matched

[leftiness]

Instead of saying something like strength: i32 = prism.east_wall_strength, do something more like this:

match prism.east {
  Unbreakable => println!("can't break"),
  Breakable(strength) => println!("{}", strength),
}

That does a better job of explaining the idea than setting the strength to -1 or something.

[leftiness]

I started on this tonight, but it quickly became tedious. See comment on #67.