Continuable.run() should report an exception as exception
Should be package internal because there's no need to expose this
Testing the behaviour of unhandled exception
unhandled exception is not resumable
Allowing Continuable to resume by throwing exception.

As a generalization, this also lets us start a brand-new program by throwing an exception, which results in an immediate abnormal termination of the program without running anything.
signify that we are not using this parameter
Subsuming YieldContinuation functionality into Continuable
HALT should set environment to null.
Having all of Continuation, Conclusion, and Continuable is just too confusing
Moving Outcome to mark it package private
Incorrect use of Outcome here