refactor: small code suggestions

[AngryMaciek]

• adjust all int and unsigned int to uint_64 or int32_t.
• conditional inclusion of MPFR only if flag provided (GPT)
• as suggested in the revision process: maybe it would be easier to merge Polynomial specialisation with the main one? just need to explicitly forbid/allow the differences...

I cannot do this; if a method is missing the compiler does not automatically fall back to the base class, the solution is what I have now: explicitly define all functions again in the specialised class. One other solution that theoretically could work is to remove the whole class specialisation and only specialise a few particular methods in the base class. This will now work here, however, because of the <MaxDeg> template parameter which is a CLASS template for the specialisation, I cannot just convert it to a FUNCTION template in the base class.