Cleanup RFC

* Remove the `Unsafe` effect.
* List all needed keywords.
* Rename `wont` to `forbid`
* Rename `Fail` to `Failure` to be gramatically correct with the use
  of `forbid`.
* Explain the `Failure` problems as discussed in the comments.
* Mention the unresolved questions about compiler inference (when it
  can't do so)