Update to version 2.0.0

This version uses an optimized internal representation of the operands which
allows to speed up some operations by a significant factor (multiplication
of large numbers is now about 40 times as fast).

This version has been tested against a number of other bc implementations
and has been found to significantly outperform them all (incl. GNU bc).

There are no functional changes in this version, it is just much faster ...

Approved by: antoine (implicit)