Macro visitor

macro_rules! visitor {
    (
        $($access:tt)? enum $name:ident, $name_mut:ident {
            Root($root:ident $(visits [$($root_host:ident),*])?),
            Branches(
                $($branch:ident $(visits [$($branch_host:ident),*])?),*
            )
        }
    ) => { ... };
}

! A macro for generating visitor types. Example:

use ::is_tree::*;
visitor! {
   pub enum Visitors, VisitorMut {
      Root(Root visits [Type1]),
      Branches(
        Branch1 visits [Type1, Type2, Type3],
        Branch2 visits [Type2, Type4],
        Branch3
      )
   }
}