Implement s-expression based communication protocol

[tsani]

The interactive mode presently communicates with emacs with an ad-hoc protocol. Instead, we should use an s-expression based scheme, which will facilitate deserialization on the Emacs end, and serialization on the OCaml end, since OCaml natively supports s-expression based serialization.

[tsani]

Given that the old-style Emacs interactive mode has been largely replaced by Harpoon, this is a low-priority issue.