Lua Generator using IR. (#6940)

* initial hack to get new Lua generator into flatc

* Starting to output enum defs for Lua

* Continue to work on table generation for Lua

* Finished basic getter access for Lua

* Added ability to get object by index

* Finished struct builder

* aliased reflection to r

* finish table builder generation

* register requiring files

* better generated header info

* Tying up loose ends

* Updated reflection to handle struct padding

* Addd type sizes to reflection

* Fixed some vector indirect issues

* Lua tests passed

* Misc cleanup

* ci fixes 1

* ci fixes 2

* renaming

* up size of type sizes

* manually ran clang-format-11 -i src/idl_parser.cpp

* fixed some windows casting

* remove stupid auto import

* more static_casting

* remove std

* update other build environments

* remove scoped enums

* replaced std::to_string with NumToString

* more win fixes

* more win fixes

* replaced old lua with new

* removed auto import

* review responses

* more style fixes

* refactor bfbs_gen_len to use code +=

* added consts

* fix lambda capture for windows

* remove unused return type

include/flatbuffers/bfbs_generator.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2021 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FLATBUFFERS_BFBS_GENERATOR_H_
+#define FLATBUFFERS_BFBS_GENERATOR_H_
+
+#include <cstdint>
+
+namespace flatbuffers {
+
+enum GeneratorStatus {
+  OK,
+  FAILED,
+  FAILED_VERIFICATION,
+};
+
+// A Flatbuffer Code Generator that receives a binary serialized reflection.fbs
+// and generates code from it.
+class BfbsGenerator {
+ public:
+  virtual ~BfbsGenerator() {}
+
+  // Generate code from the provided `buffer` of given `length`. The buffer is
+  // a serialized reflection.fbs.
+  virtual GeneratorStatus Generate(const uint8_t *buffer, int64_t length) = 0;
+};
+
+}  // namespace flatbuffers
+
+#endif  // FLATBUFFERS_BFBS_GENERATOR_H_