fork package:unexceptionalio

Mirrors forkIO, but re-throws errors to the parent thread

• Ignores manual thread kills, since those are on purpose.
• Re-throws async exceptions (SomeAsyncException) as is.
• Re-throws ExitCode as is in an attempt to exit with the requested code.
• Wraps synchronous PseudoException in async ChildThreadError.

Mirrors forkFinally, but since the body is UIO, the thread must terminate successfully or because of PseudoException