Protobackend

hooks for writing files to disk.

* `fs_hook`: dispatcher hook for saving DC_CODE, DC_SOLUTION to file
* `fs_save`: utility function to save individual files

Now can set (in this order of priority)

* DC_BACKEND_DEBUG
* SQL_BACKEND_DEBUG
* PYTHON_BACKEND_DEBUG

The WorkerThread class contains command and result queues. This allow threaded applications, like those for shell exercises, to run commands synchronously.