Convert all fns to block arg syntax.

test/break/closure_in_for.wren

@@ -1,7 +1,7 @@
 var f
 for (i in [1, 2, 3]) {
   var j = 4
-  f = fn IO.print(i + j)
+  f = new Fn { IO.print(i + j) }
   break
 }
 

test/break/closure_in_while.wren

@@ -1,7 +1,7 @@
 var f
 while (true) {
   var i = "i"
-  f = fn IO.print(i)
+  f = new Fn { IO.print(i) }
   break
 }
 

test/break/in_function_in_loop.wren

@@ -1,6 +1,6 @@
 var done = false
 while (!done) {
-  fn {
+  new Fn {
     break // expect error
   }
   done = true