Use the `with-user-abort` library to catch an interrupt signal from
`<C-c>` and exit immediately. Otherwise, the Lisp debugger is invoked,
which is not the expected behaviour for a command line program.
Tried putting the `user-abort` condition in the `handler-case` in
`main`, but it didn't appear to be caught in my tests. Decided to catch
it with `handler-case` immediately instead, confirming this works.
Unfortunately, if `<C-c>` is received before entering `main` (by running
the program and immediately pressing it), our handler won't get called,
and instead the Lisp debugger will be invoked. Not sure how to deal with
that, so I've decided not to bother.