Added support for nested flatbuffers serializing to json (#4391)

* added bool for nested_flatbuffer
setting bool flag nested_flatbuffer according to attributes
setting nested type

* added JSON serialization for nested flatbuffers

* simplified lookup of nested_flatbuffer StructDef

* added nested_flatbuffer parsing workound relying on flexbuffers

* moved nested flatbuffer parsing into its own function for clarity
removed flexbuffers workound to simplify and speed up nested_flatbuffer parsing
added support for 'legacy' nested_flatbuffers, ubyte arrays

* inlined functions for nested parser init/clean since only used once

* whitespace formatting

* changed type of FieldDef.nested_flatbuffer from bool to StructDef*
removed subsequent type lookups

* removed copies of unrequired data when initializing nested parser

* applied changes requested by reviewer

* removed superfluous lookup of nested_flatbuffer field attributes

* renamed camelCased variables to snake_case

include/flatbuffers/idl.h

@@ -232,7 +232,7 @@ struct Definition {
 
 struct FieldDef : public Definition {
   FieldDef() : deprecated(false), required(false), key(false),
-               flexbuffer(false), padding(0) {}
+               flexbuffer(false), nested_flatbuffer(NULL), padding(0) {}
 
   Offset<reflection::Field> Serialize(FlatBufferBuilder *builder, uint16_t id,
                                       const Parser &parser) const;
@@ -245,6 +245,7 @@ struct FieldDef : public Definition {
   bool native_inline;  // Field will be defined inline (instead of as a pointer)
                        // for native tables if field is a struct.
   bool flexbuffer; // This field contains FlexBuffer data.
+  StructDef *nested_flatbuffer; // This field contains nested FlatBuffer data.
   size_t padding;  // Bytes to always pad after this field.
 };
 
@@ -602,6 +603,9 @@ private:
   FLATBUFFERS_CHECKED_ERROR ParseVectorDelimiters(
       size_t &count, ParseVectorDelimitersBody body, void *state);
   FLATBUFFERS_CHECKED_ERROR ParseVector(const Type &type, uoffset_t *ovalue);
+  FLATBUFFERS_CHECKED_ERROR ParseNestedFlatbuffer(Value &val, FieldDef *field,
+                                                  size_t parent_fieldn,
+                                                  const StructDef *parent_struct_def);
   FLATBUFFERS_CHECKED_ERROR ParseMetaData(SymbolTable<Value> *attributes);
   FLATBUFFERS_CHECKED_ERROR TryTypedValue(int dtoken, bool check, Value &e,
                                           BaseType req, bool *destmatch);