Make BigInteger.arithmetic public

[andreypfau]

Motivation:

I need a method to get the length of a number in bits (BigIntegerArithmetic.bitLength), besides this interface is public, but there are no ways to get an instance except through this constant

[CLAassistant]

All committers have signed the CLA.

[ionspin]

Hi @andreypfau, thank you for your contribution! My initial intuition is that this would be better handled if there was a function bitLenght() in BigInteger that returns number of bits instead of exposing the BigIntegerArithmetic which works on ULong arrays.

The function would just call the arithmetic like in this line

Would that work for you?

[andreypfau]

Yep, That would be great

[ionspin]

Completed in #255