Dune RPC Usability Improvements

[tjdevries]

This is primarily motivated by my work on a vite plugin to integrate melange/dune with vite, vite-plugin-ocaml.

Desired Behavior

It would be great if I could control Dune RPC from a language that is not OCaml - but the RPC is not specified in a way that makes it easy to send messages from anything besides the Dune client example.

However, I have run into a few problems:

1. Dune depends on Unix, I cannot compile the example client with Melange
2. There is no RPC types package that is separate from the rest of the Dune project, so I cannot only get the RPC types and their serialization methods separately to write my own client
3. The RPC messages are all sent in S-expressions, with no options for other common serialization formats like JSON which might be a bit easier to send from other languages/runtimes

Basically, I would really like to be able to either:

• Build the client for melange, and then control it myself. Or
• Get the types and serializers separately, so that I could control the dune build process without the example client

(tagging @sabine, since we were discussing this on Discord).

Let me know if there is any additional information you need from me, or if there is some way to build/use/export the types & serializers that I wasn't aware of.

Thanks!