Avoid heap allocation for initial value of vector longhands which do not allow empty

[upsuper]

So far there are various vector longhands which have allow_empty being false. The initial value of those properties would always need a heap allocation for the only element in the vector. This should be fixed.

Gecko avoids this via having the first element stored inline. Servo can probably use this method as well.

cc @emilio @Manishearth

[Manishearth]

These should use Option<Box<T>> (or Either<None, Box<T>>) explicitly.

[upsuper]

That probably makes things more complicated. It should just be

struct AutoVec<T> {
  first: T,
  rest: Vec<T>,
}

and you can implement index trait on it.

[highfive]

cc @emilio

[bholley]

We have SmallVec for this purpose.

[nox]

I'm working on things that are similar to this.

I think we should go for:

enum ToBeNamed<T> {
    Single(T),
    Many(Box<[T]>),
}

This is smaller than SmallVec because we don't need to care about any space wasted for the vector capacity or the number of inlined items.

[nox]

In the opposite direction, I just made this for properties that can be empty and have an empty initial value.