No set

@DanielG I have used this patch set at least for one week and it seems stable. Please merge them and make a new release.

- if augen files are missing, ghc-modi automatically generates them
- dist/setup-config is re-generated if out of date
- ghc-mod-0.cache is re-generated if out of data
- A session is restarted if the timestamp of <project.cabal> changes.
- A session is restarted if the timestamp of package.cache changes.
`World` is a set of timestamps. When ghc-modi starts a new session, it memorizes the current value of `World`. If this value is changed, ghc-modi restarts another new session.