Fix timeout by ensuring signal exceptions are not erroneously caught

Fixes #7430 and fixes #10968

This commit makes the following changes:
- Add an exception `Signal` used to convert OCaml signals to exceptions.
`Signal` is registered as critical in `CErrors` to avoid being caught in the
wrong `with` clauses.
- Make `Control.timeout` into a safer interface based on `option` instead of
exceptions.
- Modify `tclTIMEOUT` to fail with `CErrors.Timeout` instead of
`Logic_monad.Tac_timeout`, as was already advertised in the ocamldoc documentation.
- Removes `Logic_monad.Tac_timeout` altogether because it no longer has a use.

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