TODO:
  * Make flat stem support default
  * implement regular polygon for skin extrusions
  * switch to skin-shaped extrusions by default
  * kailh choc has a non-square key unit; should I get that working for layouts etc? 
  * move everything over to layouts requiring a child key
  * add an "errors" or "warnings" or "suggestions" echo section in key.scad, right when the key is being made, so the errors don't get lost / repeated