[math] Implement cubic field extensions

[irakliyk]

This will let us generate proofs at 128-bit security level even in small fields (e.g. ~64 bits).

For the 62-bit field we are currently using, the irreducible polynomial could be: x³ - x + 2. But this won't work for our 128-bit field. This is not a problem per se, because we don't need to do cubic extensions of 128-bit fields, but it might be a good idea to investigate approaches of tying extension fields to specific base field (rather than giving one generic implementation for all extensions of a given degree).