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/io/write.wren

@@ -1,5 +1,5 @@
 class Foo {
-  toString { return "Foo.toString" }
+  toString { "Foo.toString" }
 }
 
 // Calls toString on argument.