understand writable as an annotation

~~~.rb
class Toto
   var x: Int writable = 5 # before
   var y: Int = 5 is writable # after
~~~

This should become the new way, so while an active migration is not needed yet, once this PR is merger, new code should use the new notation.

Bonus: writable accepts an optional agrument that is the name of the writer.
This allows the reserve the name `foo=` for high-level setters.

Eg.
~~~.rb
class Toto
   var x: Int is private writable(set_x) # generates `set_x` instead of `x=`
   fun x=(v: Int) do # `x=` is feee, so I can use it for a distinct method
      assert x >= 0
      set_x(v) # I use the generated `set_x` as a private internal setter
   end
end
~~~