Macro fadroma::stateful

macro_rules! stateful {
    (
        $Obj:ident ($($storage:tt)+): /*{ $($accessors:tt)* } no traits no accessors */
        $Readonly:ident { $($readonlies:tt)* }
        $Writable:ident { $($writables:tt)* }
    ) => { ... };
}

Because Rust can’t yet genericize over mutability, this macro can be used to implement the same readonly methods twice - once for &S and for &mut S.