Export external items as interfaces

[W95Psp]

External Rust items have no special treatment in the frontend. We need to collect them to produce interface files in the various backends so that one can provide models in the backends for external crates. For instance, Hacspec_lib is an external crate, and we would like Circus to generate an interface each backend should implement.

TODO:

• [ ] the frontend should collect every item signature for every external crate;
• [ ] the frontend should mark which external item is used by the exported crate;
• [ ] have an interface generation mode for backends.

[W95Psp]

Progress have been made with PR #743

[github-actions[bot]]

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