Emacs repl sub prompt problems

When using the Carp repl via emacs mode, the "sub prompt" (which currently is five spaces, defined in Repl.hs, line 130) is echoed leading to buggy/messy output, like this:

```
鲤
鲤
```
The alignment issue comes from evaluating a form with line breaks, which activates the sub prompt (printing a bunch of spaces). The idea behind the sub prompt in a normal repl (in the terminal) is to let the user enter multi line expressions in a nice looking way.

In `inf-carp-mode.el` (in the `emacs` dir of the Carp repo) there are two variables with regexps that are supposed to match and remove these spaces:

```elisp
inf-carp-prompt
inf-carp-subprompt
```

But for some reason they don't work. If any emacs expert has the time to look into this it would be great!

Read emacs/inf-carp-mode.el and Repl.hs around line 130, then reproduce a multiline form through Emacs Carp REPL mode. Inspect inf-carp-prompt and inf-carp-subprompt while checking how the REPL emits the sub prompt. Done means multiline output no longer echoes the five-space sub prompt or becomes misaligned.

Written by the indexing model from the issue text.