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")

test/fiber/yield_with_no_caller.wren

@@ -1,5 +1,6 @@
 var a = new Fiber {
-  Fiber.yield() // expect runtime error: No fiber to yield to.
+  IO.print("before") // expect: before
+  Fiber.yield()
 }
 
 // Run a chain of fibers. Since none of them are called, they all get discarded
@@ -7,3 +8,4 @@ var a = new Fiber {
 var b = new Fiber { a.run() }
 var c = new Fiber { b.run() }
 c.run()
+IO.print("not reached")

test/fiber/yield_with_value_from_main.wren

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

test/fiber/yield_with_value_with_no_caller.wren

@@ -1,5 +1,6 @@
 var a = new Fiber {
-  Fiber.yield(1) // expect runtime error: No fiber to yield to.
+  IO.print("before") // expect: before
+  Fiber.yield(1)
 }
 
 // Run a chain of fibers. Since none of them are called, they all get discarded
@@ -7,3 +8,4 @@ var a = new Fiber {
 var b = new Fiber { a.run() }
 var c = new Fiber { b.run() }
 c.run()
+IO.print("not reached")