refactor(pumpkin-core): Store nogood index in arena

[ImkoMarijnissen]

Stores the NogoodIndex with the nogood in the arena allocator, rather than keeping a separate HashMap. The MiniZinc score appears to be slightly improved, while the average primal integral degrades slightly.

Overall

main

{'ERROR': 5,
 'OOM': 6,
 'OPTIMAL': 100,
 'SATISFIABLE': 175,
 'SATISFIABLE_WITH_OOM': 8,
 'UNKNOWN': 95,
 'UNSATISFIABLE': 4}

new

{'ERROR': 5,
 'OOM': 5,
 'OPTIMAL': 100,
 'SATISFIABLE': 175,
 'SATISFIABLE_WITH_OOM': 8,
 'UNKNOWN': 96,
 'UNSATISFIABLE': 4}

MiniZinc Scoring

{
    `main`: 142.74,
    `new`: 150.26
}

Average Primal Integral

{
    `main`: 21.79,
    `new`: 22.35
}