feat: allow passing initial value to fiber function parameter on first call

When a fiber is started for the first time via call() or transfer(), the value argument is now bound to the fiber's function parameter if it takes one. Previously, the first value was always ignored. Also adds runtime error for fibers created with functions taking more than one parameter, and updates documentation and tests accordingly.

src/vm/wren_core.c

@@ -53,14 +53,20 @@ DEF_PRIMITIVE(fiber_new)
 {
   if (!validateFn(vm, args[1], "Argument")) return false;
 
-  ObjFiber* newFiber = wrenNewFiber(vm, AS_CLOSURE(args[1]));
+  ObjClosure* closure = AS_CLOSURE(args[1]);
+  if (closure->fn->arity > 1)
+  {
+    RETURN_ERROR("Function cannot take more than one parameter.");
+  }
+  
+  ObjFiber* newFiber = wrenNewFiber(vm, closure);
 
   // The compiler expects the first slot of a function to hold the receiver.
   // Since a fiber's stack is invoked directly, it doesn't have one, so put it
   // in here.
   newFiber->stack[0] = NULL_VAL;
   newFiber->stackTop++;
-
+  
   RETURN_OBJ(newFiber);
 }
 
@@ -106,9 +112,20 @@ static bool runFiber(WrenVM* vm, ObjFiber* fiber, Value* args, bool isCall,
   // need one slot for the result, so discard the other slot now.
   if (hasValue) vm->fiber->stackTop--;
 
-  // If the fiber was paused, make yield() or transfer() return the result.
-  if (fiber->stackTop > fiber->stack)
+  if (fiber->numFrames == 1 &&
+      fiber->frames[0].ip == fiber->frames[0].closure->fn->code.data)
   {
+    // The fiber is being started for the first time. If its function takes a
+    // parameter, bind an argument to it.
+    if (fiber->frames[0].closure->fn->arity == 1)
+    {
+      fiber->stackTop[0] = hasValue ? args[1] : NULL_VAL;
+      fiber->stackTop++;
+    }
+  }
+  else
+  {
+    // The fiber is being resumed, make yield() or transfer() return the result.
     fiber->stackTop[-1] = hasValue ? args[1] : NULL_VAL;
   }
 

src/vm/wren_value.c

@@ -170,7 +170,7 @@ ObjFiber* wrenNewFiber(WrenVM* vm, ObjClosure* closure)
 
 void wrenResetFiber(WrenVM* vm, ObjFiber* fiber, ObjClosure* closure)
 {
-  // Push the stack frame for the function.
+  // Reset everything.
   fiber->stackTop = fiber->stack;
   fiber->openUpvalues = NULL;
   fiber->caller = NULL;