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/constructor/new.wren

@@ -3,7 +3,7 @@ class Foo {
   new(a) { IO.print(a) }
   new(a, b) { IO.print(a + b) }
 
-  toString { return "Foo" }
+  toString { "Foo" }
 }
 
 // Can overload by arity.