Remove OptNewVector and use Vec instead.

Before we had Vec<T>, Option<~[T]> was used as an optimization of ~[T]
to avoid allocating for empty vectors when that was the common case.
Vec<T> itself does this optimization, so there is no need for this anymore.
auto merge of #2432 : SimonSapin/servo/remove-optnewvector, r=larsbergstrom

Before we had Vec<T>, Option<~[T]> was used as an optimization of ~[T] to avoid allocating for empty vectors when that was the common case. Vec<T> itself does this optimization, so there is no need for this anymore.

r? @larsbergstrom