Convert all fns to block arg syntax.

test/field/closure.wren

@@ -2,11 +2,11 @@ class Foo {
   new { _field = "Foo field" }
 
   closeOverGet {
-    return fn { return _field }
+    return new Fn { _field }
   }
 
   closeOverSet {
-    return fn { _field = "new value" }
+    return new Fn { _field = "new value" }
   }
 }
 

test/field/in_fn_in_static_method.wren

@@ -1,5 +1,5 @@
 class Foo {
   static bar {
-    fn _field = "wat" // expect error
+    new Fn { _field = "wat" } // expect error
   }
 }