A marker trait for sensitive information.
This will compile error:
```rust
struct Password(SensitiveData
fn posttweet
fn main() { let pwd = Password(SensitiveData::new("wutdedogdoin1!".to_string())); let msg = "Hello, world! Beep boops aggressively";
post_tweet(pwd, msg); // Uh oh, we flipped the message and the password!
}
But `NonSensitive` comes to the rescue!
Instead of posting our twitter password, we get an error.
Error message:
the trait bound sensitive_trait::SensitiveData<String>: sensitive_trait::NonSensitive
is not satisfied in Password
```