chore: ensure logical and consistent groupings of cheats in Vm.sol

[mds1]

Ref #459

In general, Vm.sol is grouped logically (e.g. all filesystem cheats in one place), but we should:

1. Make sure all groupings are correct and fix any out-of-place cheats
2. Add comments to more clearly separate sections so it's more clear

[ruvaag]

Following up from the referenced issue, I can take this up

[mds1]

Thanks @ruvaag, though I already opened a PR for this in #464 :)