for `~str`/`~[]`.
Note that `~self` still remains, since I forgot to add support for
`Box<self>` before the snapshot.
How to update your code:
* Instead of `~EXPR`, you should write `box EXPR`.
* Instead of `~TYPE`, you should write `Box<Type>`.
* Instead of `~PATTERN`, you should write `box PATTERN`.
[breaking-change]
fail!() used to require owned strings but can handle static strings
now. Also, it can pass its arguments to fmt!() on its own, no need for
the caller to call fmt!() itself.
