Remove unnecessary "static" field on program.json

We don't need this now that each load item has its own staticDir.
Add functions for reading text or binary assets.

Assets are namespaced by package, so packages can only read their own files.
Sort of WIP... To do:
1.) Don't allow ../ in file paths?
2.) Async versions of the API
3.) Make this API available on the client?
4.) More tests