improve win32 compatibility and make it easier to define custom Process modules

What

This MR improves Win32 compatibility and makes it easier to write custom versions of the Process module.

It is adapted from !57 (closed) with the following differences:

• added / tweaked documentation and comments
• removed echo from the name of the functions of the Echo module
• added changelog items

Why

#33

How to Test the MR Manually

dune runtest and the CI should still pass.

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.