Define Box.get

Folks frequently use `.get` on `Box`, triggering the implicit `Box`->`Option` conversion
and thus both making it easier to open the box (bad) and making that process slower
(baddd).

Let's define a `get` method on `Box` that is declared to return an internal class and
throws an Exception. This will fail compiler checks and also not work at runtime. For
2.6, we can define a `get` method that works like `open_!` and immediately deprecate it.