This and the `fn term_signal()` shouldn't exist. I don't think that there's a fantastic way to use these. I don't think that operations on exit codes/term signals are going to be super common, so it seems like these functions will encourage accidentally forgetting about one case or the other.

The enum values are public, so those should always be used to match against the return value of a process instead of manually calling `if status.exit_code().is_some()` and such.