Add fiber features needed for to write an external scheduler:

- Allow yielding the main fiber. This exits the interpreter. The
  host can resume it by calling a method.
- Allow getting a reference to the current fiber.

test/fiber/yield_from_main.wren

@@ -1 +1,3 @@
-Fiber.yield() // expect runtime error: No fiber to yield to.
+IO.print("before") // expect: before
+Fiber.yield()
+IO.print("not reached")