Convert all fns to block arg syntax.

test/for/close_over_loop_variable.wren

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

test/for/closure_in_body.wren

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

test/for/only_evaluate_sequence_once.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   IO.print("evaluate sequence")
   return [1, 2, 3]
 }

test/for/return_closure.wren

@@ -1,6 +1,6 @@
-var f = fn {
+var f = new Fn {
   for (i in [1, 2, 3]) {
-    return fn IO.print(i)
+    return new Fn { IO.print(i) }
   }
 }
 

test/for/return_inside.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   for (i in [1, 2, 3]) {
     return i
   }