Make callers of `${Derived}::${base}()` use casts instead.
Remove UIEvent.event().
Remove HTMLElement.element().
Remove Node.eventtarget().
Remove Element.node().
auto merge of #4130 : saneyuki/servo/cast, r=Manishearth

Fix #4124

This also introduce `BarCast::from_actual()` which is used for up-cast for dom's actual data types (non JS pointer values).