The extended_deref method will take a JSRef<'a, T> and produce an &'a T,
in contrast to the standard deref method that takes an &'b JSRef<'a, T>
and produces an &'a T. This is useful when avoiding passing a JSRef by
reference.
Replace &JSRef with JSRef in the bulk of the generated code. This will
remove a level of indirection throughout all DOM code.
This patch doesn't change methods implemented on JSRef<T> to take `self`
rather than `&self`, and it leaves a few other uses of &JSRef, but those
changes can be made incrementally.
For safe wrappers over JS<T> (eg Temporary<T>) use #[allow(unrooted_must_root)].
For all other types containing a #[must_root] value, annotate the type with #[must_root] to ensure that it is never used unrooted
