Use ptrdiff_t in repeat_n_view to satisfy weakly_incrementable

[scuzqy]

Hi @TartanLlama
I was having trouble using tl::repeat_n with msvc.
An MRE is:

// Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35728 for x64
  struct Vec3f {
    float data[3];
  };
  Vec3f value{1.0f, 2.0f, 3.0f};
  
  auto view = tl::views::repeat_n(Vec3f{1, 2, 3}, 5);
  std::vector<Vec3f> result;
  std::ranges::copy(view.begin(), view.end(), std::back_inserter(result));     // error C3889: call to object of class type 'std::ranges::_Copy_fn': no matching call operator found

Chain of requirements:
copy -> input_or_output_iterator -> weakly_incrementable -> signed-integer-like