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/closure.wren

@@ -1,9 +1,6 @@
 class Foo {
-  getClosure {
-    return new Fn { toString }
-  }
-
-  toString { return "Foo" }
+  getClosure { new Fn { toString } }
+  toString { "Foo" }
 }
 
 var closure = (new Foo).getClosure