Unify body handling.

Blocks, functions, and methods now have the same code for handling
their bodies.

This means that single-line methods work like single-line functions:
they return the result of their expression.

test/class/name_inside_body.wren

@@ -7,7 +7,7 @@ class Foo {
     IO.print(Foo.toString)
   }
 
-  static toString { return "Foo!" }
+  static toString { "Foo!" }
 }
 
 Foo.sayName // expect: Foo!