Vec<Temporary<T>> is unsafe

[jdm]

It looks appealing, but Temporary<T> only works on the stack. There's only one instance of this right now - AttributeHandlers::get_attributes.

[Manishearth]

This is easy to lint.

How are we supposed to return a list of elements, though?

[jdm]

The only solution I've come up with so far is a WebIDL-based JSVec<T> that is equivalent to NodeList but for all Reflectable elements. Unfortunately I suspect our codegen could have problems with the type parameter.

[nox]

Duplicate of #4335.