Simple Cancelation Token

Summary

This is a very simple implementation of an inter-thread cancelation token. It is modeled after tokio_util::sync::CancelationToken and uses cloning for passing it to other threads vs. having multiple separate structs with different functions. It's for when you don't need all of tokio and you just want to signal worker threads to stop, for example, after hitting Ctrl+C.

Plus, it also uses the American spelling of cancelation just to be different.