Annotation `nosuper` to avoid automatic super call in constructors

A super-init-call is automatically added in constructors that do not invoke one (a la java)
But sometimes one may want to avoid calling a super-init.

Before, this could be done with a dead super

~~~
init do
  if false the super # a super is present, so no additional implicit super-init
  rest
end
~~~

Now, one can just write

~~~
init is nosuper do
   rest
end
~~~

that is more KISS I think.