Make a `hash` `noexcept`

Reason

Users of hashed containers use hash indirectly and don't expect simple access to throw. It's a standard-library requirement.

Example, bad

template<>
struct hash<My_type> {  // thoroughly bad hash specialization
    using result_type = size_t;
    using argument_type = My_type;

    size_t operator()(const My_type & x) const
    {
        size_t xs = x.s.size();
        if (xs < 4) throw Bad_My_type{};    // "Nobody expects the Spanish inquisition!"
        return hash<size_t>()(x.s.size()) ^ trim(x.s);
    }
};

int main()
{
    unordered_map<My_type, int> m;
    My_type mt{ "asdfg" };
    m[mt] = 7;
    cout << m[My_type{ "asdfg" }] << '\n';
}

If you have to define a hash specialization, try simply to let it combine standard-library hash specializations with ^ (xor). That tends to work better than "cleverness" for non-specialists.

Enforcement

• Flag throwing hashes.