Make several parts of the code base const-correct

[user202729]

about gsl_rng_type: reading gsl's source code, we see that the objects are declared with const. Therefore, the type declaration in the pxd file need to be adapted.

About mpz_srcptr: this is just mpz_struct const*, while mpz_t in this context is just mpz_struct*. Since the functions does not modify the input argument anyway, it is better to make the argument const so that they are correctly usable in more places.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit dad6773; changes) is ready! 🎉
This preview will update shortly after each push to this PR.