segfault (use after free) when accessing solution after model is dropped

[Nudelmeister]

use russcip::prelude::*;

fn main() {
    // Create model
    let mut model = Model::new()
        .hide_output()
        .include_default_plugins()
        .create_prob("test")
        .set_obj_sense(ObjSense::Maximize);

    // Add variables
    let x1 = model.add_var(0., f64::INFINITY, 3., "x1", VarType::Integer);
    let x2 = model.add_var(0., f64::INFINITY, 4., "x2", VarType::Integer);

    // Add constraints
    model.add_cons(
        vec![x1.clone(), x2.clone()],
        &[2., 1.],
        -f64::INFINITY,
        100.,
        "c1",
    );
    model.add_cons(
        vec![x1.clone(), x2.clone()],
        &[1., 2.],
        -f64::INFINITY,
        80.,
        "c2",
    );

    let sol = model.solve().best_sol().unwrap(); // Temporary value returned from `model.solve()` is dropped.

    dbg!(sol); // <-- Segfault here
}

From looking over the russcip code the cause seems obvious. ScipPtr's drop impl is responsible for cleaning up and freeing, but Solution also holds the same raw pointer to Scip, causing a use after free.

My first thought for a fix would be to remove the pointer aliasing, only ScipPtr should hold the pointer to Scip. ScipPtr could then be reference counted. Alternatively, change the API so that it more closely mimics that of scip and require passing the ScipPtr separately.