Add a filesystem

[jathak]

In preparation for the editor (#21), I'd like to add some sort of unified file system.

Ideally, it should support files from the following sources, accessible through a unified API and mountable at any level:

• Locally saved files created by the user
• Libraries, demos, and themes in the web/scm directory
• Application-defined constants
• User-configurable "mounts" of outside sites (similar to the web/scm directory, but hosted elsewhere)
• A user's Ok backups (#22)
• The user's REPL history (concept would be that that history items would all be stored in a directory, with each item represented as a file)

Of these, only the first three would be especially important to start, but I want to make sure the system is set up to allow for the others. Mounts of outside sites could also be useful for things like lecture code.

I think I'll likely create a separate package for this, and then depend on it here.

[jathak]

I'm going to scale the scope of this issue back significantly to include only local, user-created files and built-in libraries, demos, and themes.