Remove language support for BigInteger, and move to long as internal int type for SInteger

- adopt primitive design as used in RPySOM
- this 'refactoring' is a reimplementation of the arithmetic primitives, and contains many improvements, is more complete, and consistent than before
- update core-lib to the version without BigInteger

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Need Java8 for latest Math methods.

Signed-off-by: Stefan Marr <git@stefan-marr.de>