debug: Quit on ctrl-D

[langston-barrett]

From @RyanGlScott:

Many REPLs (including GHCi and Python's REPL) interpret Ctrl-D as EOF and quit in response. It would be handy if the Crucible debugger could do the same.

isocline doesn't make this entirely straightforward to accomplish. At first glance, readlineExMaybe seems like what we'd want, as that causes Ctrl-D to return Nothing instead of empty text. On the other hand, Ctrl-C also returns Nothing, and I doubt that we'd want the debugger to exit on Ctrl-C (neither GHCi nor Python do this). See also https://github.com/daanx/isocline/issues/16.isocline doesn't make this entirely straightforward to accomplish. At first glance, readlineExMaybe seems like what we'd want, as that causes Ctrl-D to return Nothing instead of empty text. On the other hand, Ctrl-C also returns Nothing, and I doubt that we'd want the debugger to exit on Ctrl-C (neither GHCi nor Python do this). See also daanx/isocline#16.