use new slab list

[jkarneges]

This replaces all uses of our original Slab-based List with its generics-based replacement, SlabList.

As discussed earlier, the implementation is nearly the same and it performs similarly:

slab-push-pop-x10k      time:   [86.493 µs 86.659 µs 86.870 µs]
gen-slab-push-pop-x10k  time:   [83.906 µs 83.918 µs 83.931 µs]