This implements the parts of the "prepare a script element" algorithm that are
required for synchronous scripts. It also adds some infrastructure for future
support of the `async` and `defer` attributes.
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.
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.
