gh-41314: Make several parts of the code base const-correct
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.
### :memo: Checklist
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- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/41314
Reported by: user202729
Reviewer(s):
