Use "reasonable excuse" idea for unsafe functions.

[mlutze]

Flix will be removing the @Unsafe annotation. I propose we change our functions that use@Unsafe to instead use what I call the "reasonable excuse" model:

We add a new string parameter in which the user has to explain why what they are doing is safe. E.g.,

def unsafeGet(excuse: String, o: Option[a]): a = match o {
    case Some(x) => x
    case None => bug!("This should never happen because: '${excuse}'")
}