Move shell behind monotron-apps ABI

[thejpster]

The shell is currently baked tightly in to the OS - this means you can't test it separately.

We should split out ui.rs into a separate crate that uses monotron-app, but uses one of a number of specific 'slots' in flash. These can be written independently. You could also get it to mount the SD card and load a shell from SD automatically (autoexec.app?).

This is a similar mechanism to the BBC Micro OS.