C arrays for `HGEdge::intermediate_points`

[yakra]

• [ ] Use a C array for HGEdge::intermediate_points instead of a list or vector.
• [ ] Include a separate intermediate_count in unused space before format. I'm thinking uint16_t, though could use uint32_t if necessary.
  • [ ] What's the largest intermediate_points.size() of any existing edge?
• [ ] Any significant speedup from using raw coords (2 doubles) instead of HGVertex*?

• Proposed here; referenced here. Carving out a new issue mostly for a good place to leave this code:

  HGEdge collapse ctor

  // find which endpoints aren't the vertex we're collapsing and set them as our new
  // endpoints, and at the same time, build up our list of intermediate vertices
  auto    e1_count = edge1->intermediate_count,
      e2_count = edge2->intermediate_count;
  auto    new_p = intermediate_points = new HGVertex*[intermediate_count = e1_count+1+e2_count];
  
  if (edge1->vertex1 == vertex)
       {  vertex1 = edge1->vertex2;
      auto   old_p = edge1->intermediate_points + e1_count;
      while (old_p > edge1->intermediate_points) *new_p++ = *--old_p;
       }
  else {  vertex1 = edge1->vertex1;
      memcpy(new_p , edge1->intermediate_points , e1_count*sizeof(HGVertex*));
      new_p += e1_count;
       }
  
  *new_p++ = vertex;
  
  if (edge2->vertex1 == vertex)
       {  vertex2 = edge2->vertex2;
      memcpy(new_p , edge2->intermediate_points , e2_count*sizeof(HGVertex*));
       }
  else {  vertex2 = edge2->vertex1;
      auto   old_p = edge2->intermediate_points + e2_count;
      while (old_p > edge2->intermediate_points) *new_p++ = *--old_p;
       }

• [ ] We're down 1 line due to deleting 5 lines of debug text. Restoring them would make a cleaner commit diff. Do I prefer a clean diff or clean code? FWIW, I just now uncommented 4 of them to grep -c them.
• [ ] Consider swapping the 1st if & else for a sense of visual continuity, pattern-spotting, yadda yadda.
• [ ] std::string HGEdge::intermediate_point_string()

  { if (!intermediate_count) return " None";
  std::string line;
  char fstr[56];
  for (auto p = intermediate_points, end = p+intermediate_count; p < end; p++)
  {   auto i = *p;
      sprintf(fstr, "%.15g %.15g", i->lat, i->lng);
      line += " [" + *i->unique_name + "] " + fstr;
  }
  return line;
  }

• Other functions affected by the change, even if calls are commented out
  • [ ] HGEdge::str()
  • [ ] Anything else?
• [ ] While I'm in there, readabilty... wall of text -> whitespace, indentation, 1 big cout with a couple ternaries.