Prepare for new constructors

With new constructors, attributes without default value are considered even if a constructor is present.

So, while ugly, the following code is currently working.
~~~
class A
   var attr: Attr
   init do attr = new Attr(foo)
end
var a = new A
~~~

With the new constructors, the attribute `attr` will be part of the signature of the new,

~~~
var a = new A # Error, expected one argument `attr: Attr`
~~~

So this series do some cosmetic changes to accommodate the future new constructors.
Most of these change make senes own their own since they:

* reduce the number of line by removing useless init
* clarify the behavior of some attributes by adding a default value or a `noinit` annotation

More precisely, easy case of the previous code is usually migrated into

~~~
class A
   var attr = new Attr(foo)
end
var a = new A
~~~

And complex cases into

~~~
class A
   var attr: Attr is noinit
   init do attr = new Attr(foo)
end
var a = new A
~~~

The few remaining very complex cases (ex parser) will be migrated latter, and a temporally new annotation `old_style_init` may be created for them.