A simplified version of HashIntImp
for testing:
// hash_int based on key_hash.cc:HashIntImp (672431b)
template <typename T>
uint64_t hash_int(T val) {
constexpr uint64_t int_const = 11400714785074694791ULL;
uint64_t cast_val = static_cast<uint64_t>(val);
return static_cast<uint64_t>(BYTESWAP(cast_val * int_const));
}