Convert all fns to block arg syntax.

test/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   if (false) "no" else return "ok"
 }.call) // expect: ok

test/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   if (true) return "ok"
 }.call) // expect: ok

test/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.print(fn {
+IO.print(new Fn {
   while (true) return "ok"
 }.call) // expect: ok

test/return/in_function.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   return "ok"
   IO.print("bad")
 }

test/return/return_null_if_brace.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   if (true) { return }
   IO.print("bad")
 }

test/return/return_null_if_newline.wren

@@ -1,4 +1,4 @@
-var f = fn {
+var f = new Fn {
   return
   IO.print("bad")
 }