Allow passing a value when first starting a fiber.

If the function the fiber is created from takes a parameter, the value
passed to the first call() or transfer() gets bound to that parameter.

Also, this now correctly handles fibers with functions that take
parameters. It used to leave the stack in a busted state. Now, it's a
runtime error to create a fiber with a function that takes any more
than one parameter.

test/core/fiber/call.wren

@@ -3,5 +3,5 @@ var fiber = Fiber.new {
 }
 
 System.print("before") // expect: before
-fiber.call()       // expect: fiber
+fiber.call()           // expect: fiber
 System.print("after")  // expect: after

test/core/fiber/call_to_parameter.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {|value|
+  System.print(value)
+}
+
+System.print("before") // expect: before
+fiber.call()           // expect: null
+System.print("after")  // expect: after

test/core/fiber/call_with_value.wren

@@ -2,8 +2,6 @@ var fiber = Fiber.new {
   System.print("fiber")
 }
 
-// The first value passed to the fiber is ignored, since there's no yield call
-// to return it.
-System.print("before")    // expect: before
-fiber.call("ignored") // expect: fiber
-System.print("after")     // expect: after
+System.print("before") // expect: before
+fiber.call("value")    // expect: fiber
+System.print("after")  // expect: after

test/core/fiber/call_with_value_to_parameter.wren

@@ -0,0 +1,7 @@
+var fiber = Fiber.new {|value|
+  System.print(value)
+}
+
+System.print("before") // expect: before
+fiber.call("value")    // expect: value
+System.print("after")  // expect: after

test/core/fiber/new_wrong_arity.wren

@@ -0,0 +1 @@
+var fiber = Fiber.new {|a, b| null } // expect runtime error: Function cannot take more than one parameter.

test/core/fiber/transfer_to_parameter.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {|param|
+  System.print("a %(param)")
+}
+
+var b = Fiber.new {|param|
+  System.print("b before %(param)")
+  a.transfer()
+  System.print("b after")
+}
+
+var c = Fiber.new {|param|
+  System.print("c before %(param)")
+  b.transfer()
+  System.print("c after")
+}
+
+System.print("start") // expect: start
+
+c.transfer()
+// expect: c before null
+// expect: b before null
+// expect: a null
+
+// Nothing else gets run since the interpreter stops after a completes.

test/core/fiber/transfer_with_value_to_parameter.wren

@@ -0,0 +1,24 @@
+var a = Fiber.new {|param|
+  System.print("a %(param)")
+}
+
+var b = Fiber.new {|param|
+  System.print("b before %(param)")
+  a.transfer("from b")
+  System.print("b after")
+}
+
+var c = Fiber.new {|param|
+  System.print("c before %(param)")
+  b.transfer("from c")
+  System.print("c after")
+}
+
+System.print("start") // expect: start
+
+c.transfer("from main")
+// expect: c before from main
+// expect: b before from c
+// expect: a from b
+
+// Nothing else gets run since the interpreter stops after a completes.

test/core/fiber/yield_with_value.wren

@@ -7,8 +7,8 @@ var fiber = Fiber.new {
 }
 
 System.print(fiber.call())  // expect: fiber 1
-                        // expect: yield 1
+                            // expect: yield 1
 System.print(fiber.call())  // expect: fiber 2
-                        // expect: yield 2
+                            // expect: yield 2
 System.print(fiber.call())  // expect: fiber 3
-                        // expect: null
+                            // expect: null