Function compute_hash

src/value.cpp:560–565

template <class T>
static std::size_t compute_hash(rank<0>, const std::string& key, const T& x)
{
    std::size_t h = hash_value(key);
    hash_combine(h, x);
    return h;
}

static std::size_t compute_hash(rank<0>, const std::string& key, std::nullptr_t)
{