add(numerics): add prime number generator/iterator

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(numerics): add test to make sure that zeros of different precision mpfp numbers are the equal.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(numerics): add zero() method to all numeric_traits

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>