Move is_void() Element method together with the other struct methods,
it becomes part of ElementHelpers.
Merge pull request #3428 from Adenilson/moveIsVoidElement01

Move is_void() Element method together with the other struct methods.
More progress in the &JSRef -> JSRef conversion

Change all of the <Class>Methods traits to take `self` instead of
`&self`.
Convert various helper traits from &JSRef to JSRef

I converted them all with a few exceptions:

- Methods that were used by trait objects, since trait objects don't
  work with `self` methods.
- Methods that take an &'b JSRef<'a, T> and return an &'b. In reality,
  many (all?) could return an &'a instead, but this isn't allowed by the
  Deref trait.
- Methods that internally rely on the same issue with Deref.
- I left out the traits involved in layout entirely, even though not all
  of their methods suffer from one of the above problems.

There will probably be solutions to all of these problems in the future.
Remove some extraneous &* pairs