Convert all fns to block arg syntax.

test/variable/local_collide_with_function_parameter.wren

@@ -1,3 +1,3 @@
-fn(a) {
+new Fn {|a|
   var a = "oops" // expect error
 }

test/variable/undefined_local.wren

@@ -1,3 +1,3 @@
-fn {
+new Fn {
   IO.print(notDefined)  // expect error
 }.call