Convert all fns to block arg syntax.

test/this/closure.wren

@@ -1,8 +1,6 @@
 class Foo {
   getClosure {
-    return fn {
-      return toString
-    }
+    return new Fn { toString }
   }
 
   toString { return "Foo" }

test/this/nested_class.wren

@@ -2,7 +2,7 @@ class Outer {
   method {
     IO.print(this) // expect: Outer
 
-    fn {
+    new Fn {
       IO.print(this) // expect: Outer
 
       class Inner {

test/this/nested_closure.wren

@@ -1,8 +1,8 @@
 class Foo {
   getClosure {
-    return fn {
-      return fn {
-        return fn {
+    return new Fn {
+      return new Fn {
+        return new Fn {
           return toString
         }
       }

test/this/this_in_top_level_function.wren

@@ -1,3 +1,3 @@
-fn {
+new Fn {
   this // expect error
 }