* go: replace objAPI-generated Pack func with method
See discussion at https://github.com/google/flatbuffers/issues/5668
* go: replace generated union type UnPack func with method
Similar to discussion https://github.com/google/flatbuffers/issues/5668
But signature:
```
func AnyUnPack(t Any, table flatbuffers.Table) *AnyT
```
Becomes,
```
func (rcv Any) UnPack(table flatbuffers.Table) *AnyT
```
* Add flatc '--cpp_std' switch and sandbox for C++17 code generator
- Added 'flac --cpp_std legacy' for compatibility with old compilers (VS2010);
- Added experimental switch 'flac --cpp_std c++17' for future development;
- Added C++17 sandbox test_cpp17.cpp;
- C++ code generator generates enums with explicit underlying type to avoid problems with the forward and backward schema compatibility;
- Adjusted CMakeLists.txt, CI and generate code scripts to support of introduced '--cpp_std';
* Fix --cpp_std values: c++0x, c++11, c++17
* Add 'cpp::CppStandard' enum
* Add testing engine into test_cpp17
* Rebase to upstream/master
* Set default '--cpp-std C++0x'
* Fix code generation (--cpp_std C++11) in CMakeLists.txt
- Fix dependency declaration of grpctest target
* Revert --cpp-std for the tests from explicit C++11 to flatc default value (C++0x)
As recommended by https://golang.org/pkg/cmd/go/internal/generate/:
To convey to humans and machine tools that code is generated,
generated source should have a line early in the file that
matches the following regular expression (in Go syntax):
^// Code generated .* DO NOT EDIT\.$
