Stop using CollectionIterator in JsonSerializer

This reduces stack consumption and code size.
See  #2046

src/ArduinoJson/Collection/CollectionData.hpp

@@ -107,6 +107,10 @@ class CollectionData {
       return collection->remove(it, resources);
   }
 
+  SlotId head() const {
+    return head_;
+  }
+
  protected:
   iterator addSlot(ResourceManager*);
 

src/ArduinoJson/Json/JsonSerializer.hpp

@@ -22,16 +22,17 @@ class JsonSerializer : public VariantDataVisitor<size_t> {
   FORCE_INLINE size_t visit(const ArrayData& array) {
     write('[');
 
-    auto it = array.createIterator(resources_);
+    auto slotId = array.head();
 
-    while (!it.done()) {
-      it->accept(*this);
+    while (slotId != NULL_SLOT) {
+      auto slot = resources_->getSlot(slotId);
 
-      it.next(resources_);
-      if (it.done())
-        break;
+      slot->data()->accept(*this);
 
-      write(',');
+      slotId = slot->next();
+
+      if (slotId != NULL_SLOT)
+        write(',');
     }
 
     write(']');
@@ -41,18 +42,19 @@ class JsonSerializer : public VariantDataVisitor<size_t> {
   size_t visit(const ObjectData& object) {
     write('{');
 
-    auto it = object.createIterator(resources_);
+    auto slotId = object.head();
 
-    while (!it.done()) {
-      formatter_.writeString(it.key());
+    while (slotId != NULL_SLOT) {
+      auto slot = resources_->getSlot(slotId);
+
+      formatter_.writeString(slot->key());
       write(':');
-      it->accept(*this);
+      slot->data()->accept(*this);
 
-      it.next(resources_);
-      if (it.done())
-        break;
+      slotId = slot->next();
 
-      write(',');
+      if (slotId != NULL_SLOT)
+        write(',');
     }
 
     write('}');