No old style accessors

Old-style accessors are those in the form

~~~
readable writable var _foo: Foo
~~~

The plan is to remove them from the grammar (and the AST and modelize_property) in future PR.
This PR only update the exisiting code to remove the remaining occurences by:

* using new-style attributes, or
* keep old style attributes with explicit getters/setters if using new-style attributes has a too big impact on the performances (or is too complex to update)

Because of performance implications, old-style attribute may remains in the language to implements some intern parts.