Fibers!

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

test/fiber/resume_caller.wren

@@ -0,0 +1,19 @@
+var b = Fiber.create(fn {
+  IO.print("fiber b")
+})
+
+var a = Fiber.create(fn {
+  IO.print("begin fiber a")
+  b.run
+  IO.print("end fiber a")
+})
+
+IO.print("begin main")
+a.run
+IO.print("end main")
+
+// expect: begin main
+// expect: begin fiber a
+// expect: fiber b
+// expect: end fiber a
+// expect: end main

test/fiber/run.wren

@@ -0,0 +1,12 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+IO.print("before") // expect: before
+fiber.run          // expect: fiber
+IO.print("after")  // expect: after
+
+// TODO: Test handles error if fiber tries to run itself.
+// TODO: Test create is passed right argument type.
+// TODO: Test closing over stuff in fiber function.
+// TODO: Test running a finished fiber.

test/fiber/run_return_implicit_null.wren

@@ -0,0 +1,6 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: null

test/fiber/run_return_value.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+  return "result"
+})
+
+var result = fiber.run // expect: fiber
+IO.print(result)       // expect: result

test/fiber/run_with_value.wren

@@ -0,0 +1,9 @@
+var fiber = Fiber.create(fn {
+  IO.print("fiber")
+})
+
+// The first value passed to the fiber is ignored, since there's no yield call
+// to return it.
+IO.print("before")   // expect: before
+fiber.run("ignored") // expect: fiber
+IO.print("after")    // expect: after

test/fiber/type.wren

@@ -0,0 +1,5 @@
+var fiber = Fiber.create(fn null)
+IO.print(fiber is Fiber)      // expect: true
+IO.print(fiber is Object)     // expect: true
+IO.print(fiber is Bool)       // expect: false
+IO.print(fiber.type == Fiber) // expect: true

test/fiber/yield.wren

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

test/fiber/yield_return_value.wren

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

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