Fibers!

Still have some edge cases to implement but the basics are in
place and working now.

test/fiber/yield_with_value.wren

@@ -0,0 +1,14 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber 1")
+  Fiber.yield("yield 1")
+  IO.print("fiber 2")
+  Fiber.yield("yield 2")
+  IO.print("fiber 3")
+})
+
+var result = fiber.run // expect: fiber 1
+IO.print(result) // expect: yield 1
+result = fiber.run // expect: fiber 2
+IO.print(result) // expect: yield 2
+result = fiber.run // expect: fiber 3
+IO.print(result) // expect: null