Accomodate changes made to Gospel

This pull request aims at accomodating Cameleer with proposed changes made in a pull request in Gospel. See here: https://github.com/ocaml-gospel/gospel/pull/390. Proposed changes in this PR: - Cameleer should now not depend on a specific version of Gospel. - Propagate Gospel syntax errors to Cameleer. - Type invariants are now expressed differently. See here: [Gospel Type Specifications](https://ocaml-gospel.github.io/gospel/language/type-specifications) or ocaml-gospel/gospel/issues/369 I believe this pull request should be delayed until https://github.com/ocaml-gospel/gospel/pull/390 is merged.