Add specialization for `deque1.prepend(deque2.drain(range))` (VecDeque::prepend and extend_front) Tracking issue: rust-lang/rust#146975 This specialization makes sure `deque1.prepend(deque2.drain(..))` gets similar speed to `deque1.append(&mut deque2)`. `deque1.prepend(deque2.drain(..))` is the equivalent of `deque1.append(&mut deque2)` but appending to the front (see the [second example of prepend](https://doc.rust-lang.org/nightly/std/collections/struct.VecDeque.html#method.prepend), prepend is from the same tracking issue).