Adding GCD rings and Euclidean rings, along with instances for BigInt

[denisrosset]

This aligns the commutative rings/field hierarchy with the one available in Spire.

I haven't provided instances for non-exact types such as Long, Int, etc, because their overflow behaviour doesn't work with the laws.

(Spire tests such instances by aborting the test in case of an overflow -- which is then considered undefined behaviour).