Improved performance of mp_is_set

[joaquintides]

Crude compile times on my local machine for this testing code:

#include <boost/mp11.hpp>
using namespace boost::mp11;

constexpr int N = 16;

using L1 = mp_iota_c<N>;
using L2 = mp_iota_c<N*2>;

template<class T> using Pr = mp_is_set<mp_push_back<L1, T>>;

using R = mp_count_if<L2, Pr>;

static_assert( R::value == N, "" );

int main()
{
}