Faster hashmap

[Gullesnuffs]

The following was >2x faster than std::unordered_map for https://codeforces.com/gym/103861/problem/B and the pbds hash map was also terrible (slow construction time? too much memory use?):

typedef uint64_t ull;                                                                                                                   
template<class T>
struct HashMap {
    int b;
    vector<pair<ull, T>> v;
    HashMap(int b) : b(b), v(1 << b) {}
    T& operator[](ull x) {
        ull y = x >> (64 - b), m = (1 << b) - 1;
        while (v[y].first && v[y].first != x) ++y &= m;
        v[y].first = x;
        return v[y].second;
    }   
};

Maybe worth adding to KACTL instead of the pbds one?