Macro maplike

macro_rules! maplike {
    ( $self:ident, $internal:ident ) => { ... };
}

Usage:

pub(crate) struct TestBindingMaplike {
    // maplike<DOMString, long>
    internal: DomRefCell<IndexMap<DOMString, i32>>,
}
impl Maplike for TestBindingSetlike {
    type Key = DOMString;
    type Value = i32;
    maplike!(self, internal);
}