Writing PopLastN in this way is probably a bit of a micro-optimization,
but at least it comes with tests and some comments for verifying the
goodness of the code.
The scheme in TestSegmentedVectors to use manually-annotated points and
magic numbers corresponding to those annotations works OK for small
numbers of operations. But for testing bulk push and pop, we're going
to be doing many more operations, so let's move to recording explicitly
in code the operations we expect to see, and checking those instead.
