boost::random::splitmix64::discard: O(n) -> O(1)

[bddap]

boost::random::splitmix64::discard is potentially/probably pokey:

    /** Advances the state of the generator by @c z. */
    inline void discard(std::uint64_t z) noexcept
    {
        for (std::uint64_t i {}; i < z; ++i)
        {
            next();
        }
    }

perhaps pick this more performant procedure

    /** Advances the state of the generator by @c z. */
    inline void discard(std::uint64_t z) noexcept
    {
        _state += z * UINT64_C(0x9E3779B97F4A7C15);
    }

[bddap]

ps: clang -O2 and better does seem to figure this one out automatically, -O1 does not