Add unsafe Setters

[jnsiemer]

Description
This PR implements...

• public unsafe set_<fmpz_struct_name> for every underlying fmpz struct.

Testing

• I added basic working examples (possibly in doc-comment)

Checklist:

• I have performed a self-review of my own code
  • The code provides good readability and maintainability s.t. it fulfills best practices like talking code, modularity, ...
    • The chosen implementation is not more complex than it has to be
  • My code should work as intended and no side effects occur (e.g. memory leaks)
  • The doc comments fit our style guide

[Marcel583]

Looks good. Just a question I have and some comment changes

[Marcel583]

How does it work for multiple references?
You say that the function only works if no other references exist/are used in the comment. But is there no way to rewrite the function such that it does? Or is it just too much for us to implement right now?
I tried this, but it did not work:

/// Checks if setter works for all references.

#[test]
#[allow(unused_mut)]
fn counter_works() {
    let mut modulus_1 = Modulus::from(3);
    let mut modulus_2 = Modulus::from(3);
    let mut modulus_3 = modulus_1.clone();

    let mut flint_struct = MaybeUninit::uninit();
    let mut flint_struct = unsafe {
        fmpz_mod_ctx_init(flint_struct.as_mut_ptr(), &fmpz(2));
        flint_struct.assume_init()
    };

    unsafe { modulus_1.set_fmpz_mod_ctx(flint_struct) };

    assert_eq!(Modulus::from(2), modulus_1);
    assert_eq!(Modulus::from(3), modulus_2);
    assert_eq!(Modulus::from(2), modulus_3);
}

[jnsiemer]

I would have loved to implement this. Unfortunately, you can't change the object that a Reference Counter (Rc), which we implement our Modulus objects with, retrospectively.
As I wasn't able to find out which objects point to the Modulus object (I believe that the data-structure internally only keeps a reference from the object pointing to the Rc, which would be absolutely sensible), it could be possible that this is impossible if you don't want to do a search for a reference to that Rc and replace the pointer. Also in this case, I wouldn't know how to do it and it seems very inefficient.

Thus, I settled for this solution for now.