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]
values to be copied. Rewrite kind computation so that instead of directly
computing the kind it computes what kinds of values are present in the type,
and then derive kinds based on that. I find this easier to think about.
Fixes#4821.
