acast and dcast are generics with data.frame method.

* `cast` is not exported.
* `acast.data.frame` and `dcast.data.frame` are documented (material from `cast`).
* `?acast` and `?dcast` redirect to the `.data.frame` methods (created an alias).
* Examples are split from `cast`.
* `check()` runs without any warnings/errors.
* `dev_help("cast") doesn't work anymore.