Mike P is right on the money about that. child abuse and elderly abuse are the only situation were mandated reporters are obligated by law to report to police.
GP is only part right. You may expect a lawsuit but unless your a doctor or an
on call, practicing and medically trained first responder there is no law requiring you to stop and render assistance.
What that means is if I, an on call EMT/fire fighter were to come across a motor vehicle accident in the town in witch I work I have a legal obligation to help. however if that MVA is on the highway, let's say on the way to the cape, I could " legally" drive right by. it's called the first responders act and it also protects me if I Do decide to render assistance and some one tries to sue me. The reason for this bill is to encourage properly trained first responders to help w/o the need for liability concerns.(That's why I always stop

)
sorry for my long winded legal speech. your question Raven was "if a citizen witnesses a crime and then does not report it isn't that a crime in itself?"
The answer is supposed to be a bit vague and a lawyer may interpret the law differently. No it's not a crime to keep it to your self unless during the investigation of said crime an officer of the law asks you a direct question about the crime and you lie. That's called perjury. And yes, not answering the question is the same as lying if you know the answer to the question.
I'm not a cop or a lawyer so take what I say for what it's worth then do what you think is right.