Remove C++-specific union codegen code.
Reuse getTypes in UnionConversions().

This should be equivalent, except that the removed code ignores return
values (which we currently don't support).
Merge UnionConversions() into UnionTypes().

It's not very useful to have one but not the other, so it makes more sense
to deal with them together.
Use derived traits for Au

This code did not use derived traits previously because their methods were not
inlined, but this was fixed in mozilla/rust#10557.
Make TimeStamp in event.rs return the actual timestamp
Removed unnecessary return and moved crate import to script.rs
Webidl and basic implementation of XHR object