Win32 compatibility and thread-safety for Process

Description / Motivation

#33

How to Test the MR Manually

This is blocked for testing because of Lwt blockers discussed in the motivating issue.

But the theoretical output for Windows compatibility should be:

# Lwt_main.run (Tezt.Process.program_path "cmd.exe");;
- : string option = Some "C:\\Windows\\System32\\cmd.exe"

Here is the shexp backend that needs the thread-safety of Process (which needs a thread-safe Echo):

diskuv/share/tezt@6dae695a

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.