Gh 1186 big math log 02

A proposed implementation of BigMath.log(x, precision). Based on the MRI C implementation and improved in a few ways (mainly readability, also using window shift already for any value larger than 10 and not from 100 as in the MRI Ruby case).

Using BigMath.div and BigMath.mult (4cf295a) the precision of intermediate calculations was controlled and reasonable performance is achieved (efd3748). Performance is still slower than the MRI C implementation.