[ntuple] Handling of over-aligned types

[hahnjo]

Description

In some type-punned fields, we make assumptions that the alignment is smaller than alignof(std::max_align_t).

Reproducer

This breaks in certain combinations of types, for example a std::vector of an over-aligned type:

#include <ROOT/RNTupleModel.hxx>
#include <ROOT/RNTupleReader.hxx>
#include <ROOT/RNTupleWriter.hxx>

#include <sstream>
#include <vector>

struct alignas(64) CacheLine {
  int i = 0;
};

void ntuple_aligned() {
  auto model = ROOT::Experimental::RNTupleModel::Create();
  using Vector = std::vector<CacheLine>;
  model->MakeField<Vector>("v");

  {
    auto writeModel = model->Clone();
    auto vPtr = writeModel->GetDefaultEntry().GetPtr<Vector>("v");
    auto writer = ROOT::Experimental::RNTupleWriter::Recreate(std::move(writeModel), "ntuple", "ntuple_aligned.root");

    vPtr->push_back({});
    writer->Fill();
  }

  auto readModel = model->Clone();
  auto vPtr = readModel->GetDefaultEntry().GetPtr<Vector>("v");
  auto reader = ROOT::Experimental::RNTupleReader::Open(std::move(readModel), "ntuple", "ntuple_aligned.root");
  reader->LoadEntry(0);

  auto ptr = &vPtr->at(0);
  auto uptr = reinterpret_cast<std::uintptr_t>(ptr);
  // Do a small dance to prevent the compiler from optimizing the alignment check...
  std::stringstream ss;
  ss << uptr;
  ss >> uptr;
  std::cout << ptr << " (" << uptr << ") % 64 = " << (uptr % 64) << std::endl;
}

possible output:

0x48c9230 (76321328) % 64 = 48

[hahnjo]

Some other fields try to get this right, for example RRecordField. We should investigate if we can make std::vector<char> allocate over-aligned memory areas. The other option is to forbid over-aligned types (and possibly simplify some other code).