Macro setlike

macro_rules! setlike {
    ( $self:ident, $x:ident ) => { ... };
}

Usage:

pub(crate) struct TestBindingSetlike {
    // setlike<DOMString>
    internal: DomRefCell<IndexSet<DOMString>>,
}
impl Setlike for TestBindingSetlike {
    type Key = DOMString;
    setlike!(self, internal);
}