Computing with Big Numbers

I added some test code to the Fexl release which demonstrates some computations with big numbers.

The bn.fxl file defines addition and multiplication of big numbers, along with conversion to and from string.

The b20.fxl file runs some tests, including the computation of 2^256, a 78 digit number:

115792089237316195423570985008687907853269984665640564039457584007913129639936

The reference output is here.