Use pull-style iterators to build index and tree

[bwesterb]

Closes #49

[lukevalenta]

Since we end up doing this multiple times (here and in unmarshalOne below), might be worth moving to a separate function:

func allocate[T any]() T {
	var t T
	reflect.ValueOf(&t).Elem().Set(reflect.New(reflect.TypeOf(t).Elem()))
	return t
}

Then can have just t := allocate[T]() in the functions.

[bwesterb]

Thought about it, but we first need to check if it changes allocation/inlining behaviour. Also repeating something twice isn't that bad.