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/this/this_in_static_method.wren

@@ -4,7 +4,7 @@ class Foo {
     IO.print(this.bar) // expect: bar
   }
 
-  static bar { return "bar" }
+  static bar { "bar" }
 }
 
 Foo.test