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_method.wren

@@ -1,6 +1,6 @@
 class Foo {
-  bar { return this }
-  baz { return "baz" }
+  bar { this }
+  baz { "baz" }
 }
 
 IO.print((new Foo).bar.baz) // expect: baz