more whitespace in closure builder
add a couple helper methods to closure builder
closure builder: move body insertion around
closure builder: extract helper for setting parent scope
closure builder: set parent scope on method after method creation

before, it did scope assignment by iterating over the created methods after the klass was created
Now it does scope assignment when the method is created/cloned
closure builder: pass klass to new_closure_call instead of type future
replace if _.nil? with unless _ in factory.rb
scripts should have a void return type

Java's main method has a void type. If someone used a return in a script body, we should at least complain properly. This change makes it so that a script like

```mirah
  puts 'hello'
  return if true
  puts 'world'
```

will print hello