Rename doc/HACKING/design members to reflect current architectural division
The original tor-guts.git document pre-dated the source code re-organization when we divided things into src/lib/, src/core/, src/feature/, and src/app/.
We'll want to edit the documentation to correspond to reality. But before we do that, we'll want to rename and divide the modules so that they have the right names. (Git handles edits better if there are no edit/split/move conflicts.)