Fix up the spatialmap to have an occupied_by method which checks if a square is occupied and returns what is there.

spatialmap.cpp

@@ -32,15 +32,19 @@ void SpatialMap::move(Point from, Point to, Entity ent) {
   insert(to, ent, data.collision);
 }
 
-bool SpatialMap::occupied(Point at) const {
+Entity SpatialMap::occupied_by(Point at) const {
   auto [begin, end] = $collision.equal_range(at);
   for(auto it = begin; it != end; ++it) {
     if(it->second.collision) {
-      return true;
+      return it->second.entity;
     }
   }
 
-  return false;
+  return DinkyECS::NONE;
+}
+
+bool SpatialMap::occupied(Point at) const {
+  return occupied_by(at) != DinkyECS::NONE;
 }
 
 bool SpatialMap::something_there(Point at) const {