Use VecDeque as a queue instead of Vec

[steviez]

Problem

traverse_children_mut() searches through blocks by pushing block's children onto a queue. Popping from the front of a Vec of n items requires the remaining n-1 items to be shifted to fill the first n-1 positions. VecDeque is a ring buffer, and as such, saves the memcpy when popping from the front.

For what it is worth, this change will probably have negligible impact. The next_slots queue should never really get that large; in the absence of forking, this queue would just be a single element, with a parent getting popped off and then replaced by its' child. With normal forking (ie current epoch boundary issue), we are typically looking at on the order of 10 forks. Regardless, VecDeque is the right structure here.

Summary of Changes

Use VecDeque instead of Vec.

[yhchiang-sol]

Looks good.

Agree that the actual performance gain might be negligible, but the pop and push pattern in the code is the exact use case of VecDeque. While Vec is also a growable container, VecDeque should be no worse than Vec in this case.