Remove scary pointer arithmetic and linear search in the built-in descriptor

[Anton3]

Currently every any has a built-in descriptor support. To support it for upcasted any instances, there is a scary piece of code:

  auto* p = reinterpret_cast<const value_type*>(vtable_ptr);
  do {
    --p;
  } while (*p);
  auto* type_desc_ptr = reinterpret_cast<const std::byte*>(p) - offsetof(vtable_with_metainfo<>, terminator);
  return *reinterpret_cast<const descriptor_t*>(type_desc_ptr);

It goes back, searching for a nullptr entry in the beginning, then assumes that next goes the descriptor. We at userver think that such code is unnecessarily scary.

Descriptors can be implemented as a regular requirement, as e.g. Boost.TypeErasure does. Then no special logic will be needed to retrieve the corresponding vtable slot.

[kelbon]

removed, now RTTI is optional and enabled by aa::type_info Method