Print diagonal matrix

Reference issue

Allows users to print diagonal matrix expressions without assigning them to a dense object. Cute utility that may be useful for debugging. Surprisingly, it is remarkably flexible and incurs no allocations (provided the expression itself does not).

Example input:

int main()
{
  VectorXf a(10);
  internal::set_is_malloc_allowed(false);
  a.setRandom();
  // plain vectors
  std::cout << a.asDiagonal() << "\n\n";
  // simple expressions
  std::cout << VectorXf::EqualSpaced(10, -2.0f, 1.5f).asDiagonal() << "\n\n";
  // slightly more complicated expressions
  std::cout << MatrixXf::Random(10, 10).diagonal(-1).asDiagonal() << "\n\n";
}

Example output:

 -0.997497          0          0          0          0          0          0          0          0          0
         0   0.127171          0          0          0          0          0          0          0          0
         0          0  -0.613392          0          0          0          0          0          0          0
         0          0          0   0.617481          0          0          0          0          0          0
         0          0          0          0   0.170019          0          0          0          0          0
         0          0          0          0          0 -0.0402539          0          0          0          0
         0          0          0          0          0          0  -0.299417          0          0          0
         0          0          0          0          0          0          0   0.791925          0          0
         0          0          0          0          0          0          0          0    0.64568          0
         0          0          0          0          0          0          0          0          0    0.49321

  -2    0    0    0    0    0    0    0    0    0
   0 -0.5    0    0    0    0    0    0    0    0
   0    0    1    0    0    0    0    0    0    0
   0    0    0  2.5    0    0    0    0    0    0
   0    0    0    0    4    0    0    0    0    0
   0    0    0    0    0  5.5    0    0    0    0
   0    0    0    0    0    0    7    0    0    0
   0    0    0    0    0    0    0  8.5    0    0
   0    0    0    0    0    0    0    0   10    0
   0    0    0    0    0    0    0    0    0 11.5

-0.668203         0         0         0         0         0         0         0         0
        0   0.97705         0         0         0         0         0         0         0
        0         0 -0.108615         0         0         0         0         0         0
        0         0         0 -0.761834         0         0         0         0         0
        0         0         0         0 -0.990661         0         0         0         0
        0         0         0         0         0 -0.982177         0         0         0
        0         0         0         0         0         0  -0.24424         0         0
        0         0         0         0         0         0         0 0.0633259         0
        0         0         0         0         0         0         0         0  0.142369

What does this implement/fix?

Additional information