• Instead of storing MaybeUninit<T> in Rooted<T>, and initializing RootedGuard from a &mut Rooted<T>, store T in Rooted<T> and initialize RootedGuard from an &mut MaybeUninit<Rooted<T>>.

  Currently every time we touch the data through RootedGuard<T>, we need to re-assert to the compiler that it is initialized. Adding one more thing to think about in an already complicated safety dance. By instead getting rid of the half initialized state, and passing the entire place in as uninitialized memory, this part of initialization can happen through entirely safe functions, and access never needs to worry about it.

• Correctly mark ValueArray::get_ptr as safe - it's doing things with the pointer that might be unsafe.

  This lets us drop the unsafe block in fn from(array: &Rooted<ValueArray<N>>) -> JS::HandleValueArray

• Rename Rooted::ptr to Rooted::data, whenever working with this type via RootedGuard, we currently have a pointer to a field name ptr which is in fact not (necessarily) a pointer. This is... confusing. Let's change the name to something else.

None of these changes are necessary for correctness or anything, they would just make me happier when reading the code.

Servo still builds with this change, we're technically changing public APIs, but not ones that are actually directly used.