`DList` should have a method to push all elements from another list, leaving the other one empty

[pcwalton]

All the current ways to do this involve more moves than necessary, hurting performance. This was an important optimization in Servo.

cc @aturon @gankro

[aturon]

@pcwalton Do you mean a variant of append/prepend that does not require ownership of the other list?

[mbrubeck]

c.f. servo/servo#3831

[Gankra]

I assume the alternative right now is to make an empty list, swap them, and then append?

[aturon]

It seems like we could just replace append with this, and then drop prepend since it would no longer be needed. @gankro, agreed?

[Gankra]

Yeah this is strictly more flexible than append. Sounds good to me.

[gereeter]

Forgive my ignorance, but why should this be any faster than self.append(mem::replace(other, DList::new()))? It doesn't save any allocations, and it seems that LLVM should be able to optimize away the small (3-word), constant size moves.

[Gankra]

DList now has apppend