Performance annotations

[Lantua]

@Philippe_Hausler

It also connotes concurrency safety - withUnsafeContinuation .

I brought this up during the review:

@Joe_Groff

@Lantua

I don't remember if this has been brought up before, but I think this would be the first unsafe function when its meaning is outside of memory safety. Are we embracing a broader meaning of unsafe or should we add another "keyword"?

It is absolutely memory-unsafe to misuse a continuation.