Start of the main TUI code.

Start of the main TUI code.

So far, I've made a struct that encapsulates the Ratatui state and
some subthread comms machinery, i.e. does the physical work. Then
there's a sub-struct to contain the logical UI state (so it can be
borrowed mutably without having to re-borrow the main struct), which
gets to draw things on the screen, update itself in response to
keypresses, and tell the physical side what to do after the
keypress (options currently limited to 'beep', 'exit', or 'just redraw
and go round the loop again').

Currently the UI itself is just "hello, world". I think menus are
next.