Durable local storage proposal

This is a proposal for #598


Our design tastes differ in whether we think it is better to expose a cross product of possibilities to users, or a few important named points in that space.  So, I'm going to leave it as is and wait for others to weigh in.  

I have moved PackageDirectory, RemoteDirectory, and DebugDirectory to a "Future Work" section, so maybe what is left is less controversial.