Ensure compatibility with OCaml 5

Changes to support OCaml 5 are as follows:

• Module Stream is no longer supported.
• Module Pervasives has been renamed to Stdlib.
• Type Format.format_tag_functions is no longer supported.
• Module Ephemeron is largely crippled.

These changes should become suitable for Why3 once the minimal supported version of OCaml is bumped to 4.08.

By lack of the corresponding dependencies, some parts of Why3 have not been exercised yet:

• IDE (dep: lablgtk3)
• Plugins (dep: ocamlgraph)
• TryWhy3 (dep: js_of_ocaml)