Use the fs-mistrust crate for state, cache, and configuration

This branch uses the new fs-mistrust crate to check our cache, our persistent state, and our configuration files.

I've made the setting to disable fs-mistrust dependent on the environment, since we might not want to trust the configuration file until we know if it's editable by somebody else.

This branch required changes to the APIs for FsStateMgr and DirMgrConfig and arti::run. Let's consider whether we should try harder to make those stable.

Assigning review to @Diziet since he's bound to have thoughts about how I can structure the configuration logic better.

Closes #315 (closed)