Provide a common API across `Option` and the `Ok` and `Err` variants of `Result`.

[Rendered](https://github.com/bjz/rfcs/blob/option-result-api/active/0000-option-result-api.md)
Once you’ve got that, you’re back to the current situation with `Result.ok()` and `Result.err()`, and so the actual effect of this is simply “remove all the methods on `Result` except for `ok` and `err`.