Arena allocator

//https://lesleylai.info/en/std-align

struct Arena {
  void* ptr = 0;
  std::size_t size_remain = 0;

  [[nodiscard]]
  auto aligned_alloc(std::size_t alignment, std::size_t size) noexcept -> void*
  {
    void* res = std::align(alignment, size, ptr, size_remain);
    if (res) {
        ptr = static_cast<std::byte*>(res) + size;
        size_remain -= size;
        return res;
    }
    return nullptr;
  }
};