Create a third library for the modules that can be shared between Testo and Testo_lwt

[mjambon]

This allows sharing tags because they now have the same type. All the modules that don't depend on the Monad.t type moved to this new library.

test plan: Check that the Tag module is shared as follows:

$ dune utop
...
utop # Testo.Tag.declare "a";;
- : Testo.Tag.t = "a"

utop # Testo_lwt.Tag.declare "a";;
Exception:
Internal error in the Testo library at File "util/Tag.ml", line 44, characters 23-30: Invalid argument: Testo.declare_tag: tag "a" was declared multiple times.
Each tag must be declared exactly once to avoid accidental conflicts.

PR checklist:

• [x] Purpose of the code is evident to future readers
• [x] Tests are included or a PR comment includes a reproducible test plan
• [x] Documentation is up-to-date
• [ ] A changelog entry was added to CHANGES.md for any user-facing change

Check out CONTRIBUTING.md for more details.