Added GCD, LCM. Iterator specialisations for etl::rotate. Constexpr for etl::absolute

Features:
Added GCD and LCM algorithms. etl::gcd and etl::lcm
Added specialisations of etl::rotate for different iterator types
Added ETL_NODISCARD, ETL_CONSTEXPR14 and ETL_NOEXCEPT to etl::absolute and etl::absolute_unsigned