Add PivotRootRule class

... and tests for it.

This replaces the old code that just stores the full rule as text.

We also get rid of the old ['allow'] and ['deny'] items in ProfileStorage, the handling of old write functions, and the last usage of _Raw_Rule (and therefore _Raw_Rule itsself).

Also delete the old test-pivot_root_parse.py which relied on the ancient code, and even used a wrong syntax in its test rules.

Oh, and aa-logprof can now ask about pivot_root events.

See the individual commits for details.