message: add missing unsafeCurrentFiber()
diff: @@ -89,6 +89,8 @@ export const _tracing = new TracingContext()
  export const currentFiber = new AtomicReference<FiberContext<any, any> | null>(null)
  +export const unsafeCurrentFiber = () => O.fromNullable(currentFiber.get)
  +
  export class FiberContext<E, A> implements Fiber.Runtime<E, A> {
  readonly _tag = "RuntimeFiber"
  readonly state = new AtomicReference(initial<E, A>())
  