hash combination: Fix struct hash in sm::vec

[sebsjames]

for (std::size_t n = 1u; n < N; ++n) { h ^= std::hash<S>{}(v[n]); }

should probably read

for (std::size_t n = 1u; n < N; ++n) {
    h = h ^ (std::hash<S>{}(v[n]) << 1);
}