Make "this" the implicit receiver!

test/this/closure.wren

@@ -1,7 +1,7 @@
 class Foo {
   getClosure {
     return fn {
-      return this.toString
+      return toString
     }
   }
 

test/this/nested_class.wren

@@ -1,13 +1,13 @@
 class Outer {
   method {
-    IO.print(this.toString) // expect: Outer
+    IO.print(this) // expect: Outer
 
     fn {
-      IO.print(this.toString) // expect: Outer
+      IO.print(this) // expect: Outer
 
       class Inner {
         method {
-          IO.print(this.toString) // expect: Inner
+          IO.print(this) // expect: Inner
         }
         toString { return "Inner" }
       }

test/this/nested_closure.wren

@@ -3,7 +3,7 @@ class Foo {
     return fn {
       return fn {
         return fn {
-          return this.toString
+          return toString
         }
       }
     }