Dedupe builtin/vX/util/*

Similar to https://github.com/filecoin-project/go-state-types/pull/286, almost everything in `builtin/vX/util/*` is duplicated and identical (apart from minor import reorgs) from v8 to v15. So we should just move them to the top level. The exception to this is the `adt` directory which can stay.

I'm not sure we even need to leave aliases in place, this could be a breaking change for the next major release. If a future state change adjusts any of these (e.g. `FilterEstimate` might grow a new field?) then we could do what we do with `ActorV4` and `ActorV5` or introduce a new package with a version, e.g. `smoothing2`.