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]

But you check the getter here. So the initial comment should be correct

[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);
}

[Marcel583]

The function returns nothing. It sets the value

Suggested change

	/// Returns a mutable reference to the field `modulus` of type [`Modulus`].
	/// Sets a mutable reference to the field `modulus` of type [`Modulus`] to a given `fmpz_mod_ctx`.

[Marcel583]

Suggested change

	/// **WARNING:** The returned struct is part of [`flint_sys`].
	/// Any changes to this object are unsafe and may introduce memory leaks.
	/// Please be aware that most moduli are shared across multiple instances and all
	/// modifications of this struct will affect any other instance with a reference to this object.
	/// **WARNING:** The set struct is part of [`flint_sys`].
	/// Any changes to this object are unsafe and may introduce memory leaks.
	/// Please be aware that most moduli are shared across multiple instances and all
	/// modifications of this struct will affect any other instance with a reference to this object.

[Marcel583]

Suggested change

	/// Returns a mutable reference to the field `modulus` of type [`ModulusPolynomialRingZq`].
	/// Sets a mutable reference to the field `modulus` of type [`ModulusPolynomialRingZq`] to a given `fq_ctx_struct`.