add the annotation `readonly` on attributes

`readonly` just generate no setter. So the value must be set trough the initial value.

Basically it is like the Scala `val` keyword.
Note that unlike the Java `final` modifier, the value cannot be set in a constructor (use a private setter instead if you need a restriction on the setter visibility)

~~~.rb
class Toto
   var a = new Array[Int] is readonly
end
var t = new Toto
t.a.add 5 # OK
t.a = new Array[Int] # Error: no `a=` in Toto
~~~