Improve handling of `(push)` and `(pop)` (#8641)

Improve handling of `(push)` and `(pop)` (#8641)

This extends PushCommand and PopCommand to take a number of levels
to push/pop. We have support for pushing an arbitrary number of levels
at the API level, so this simplifies the parser code and makes dumping
more precise (previously, we were dumping (push 2) as two (push 1)
commands).