Split the variable dump and stdout in Prolog format toplevel

[guregu]

Currently the "prolog" format toplevel just returns a string. If you write to stdout, this gets mixed with the actual variable dump / residual goals output of the toplevel.

Maybe it should return an object like this instead:

{answer: "X=1.", stdout: "hello world", stderr: "Warning: ..."}

It's possible to capture this output like the JSON format, but it introduces a spurious choice point. Some hacking of the toplevel code in Trealla could spit out some separators for us. I don't want to mess with it too much as I would like to upstream my fork eventually. Maybe a CLI flag.