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.

doc/site/modules/core/fiber.markdown

@@ -29,6 +29,11 @@ fiber is run. Does not immediately start running the fiber.
       System.print("I won't get printed")
     }
 
+`function` must be a function (an actual [Fn][] instance, not just an object
+with a `call()` method) and it may only take zero or one parameters.
+
+[fn]: fn.html
+
 ### Fiber.**suspend**()
 
 Pauses the current fiber, and stops the interpreter. Control returns to the
@@ -97,46 +102,36 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
 
 ### **call**()
 
-Starts or resumes the fiber if it is in a paused state.
+Starts or resumes the fiber if it is in a paused state. Equivalent to:
 
     :::wren
-    var fiber = Fiber.new {
-      System.print("Fiber called")
-      Fiber.yield()
-      System.print("Fiber called again")
-    }
-
-    fiber.call() // Start it.
-    fiber.call() // Resume after the yield() call.
-
-When the called fiber yields, control is transferred back to the fiber that
-called it.
-
-If the called fiber is resuming from a yield, the `yield()` method returns
-`null` in the called fiber.
-
-    :::wren
-    var fiber = Fiber.new {
-      System.print(Fiber.yield())
-    }
-
-    fiber.call()
-    fiber.call() //> null
+    fiber.call(null)
 
 ### **call**(value)
 
-Invokes the fiber or resumes the fiber if it is in a paused state and sets
-`value` as the returned value of the fiber's call to `yield`.
+Start or resumes the fiber if it is in a paused state. If the fiber is being
+started for the first time, and its function takes a parameter, `value` is
+passed to it.
+
+    :::wren
+    var fiber = Fiber.new {|param|
+      System.print(param) //> begin
+    }
+
+    fiber.call("begin")
+
+If the fiber is being resumed, `value` becomes the returned value of the fiber's
+call to `yield`.
 
     :::wren
     var fiber = Fiber.new {
-      System.print(Fiber.yield())
+      System.print(Fiber.yield()) //> resume
     }
 
     fiber.call()
-    fiber.call("value") //> value
+    fiber.call("resume")
 
-### **error***
+### **error**
 
 The error message that was passed when aborting the fiber, or `null` if the
 fiber has not been aborted.
@@ -154,8 +149,6 @@ fiber has not been aborted.
 Whether the fiber's main function has completed and the fiber can no longer be
 run. This returns `false` if the fiber is currently running or has yielded.
 
-### **transfer**()
-
 ### **try**()
 Tries to run the fiber. If a runtime error occurs
 in the called fiber, the error is captured and is returned as a string.
@@ -170,6 +163,8 @@ in the called fiber, the error is captured and is returned as a string.
 
 If the called fiber raises an error, it can no longer be used.
 
+### **transfer**()
+
 **TODO**
 
 ### **transfer**(value)