FlexBuffers are actually completely unrelated to FlatBuffers and can be used separately.
However, since they utilize several utility functions from flatbuffer.h, they require this header file, which creates unreasonable dependency.
By moving those utility functions to separate base header, both libraries can use same code but still be independent
* Pedantic Visual Studio build (/W4 /WX)
* Pedantic Visual Studio build (/W4 /WX)
* Pedantic Visual Studio build (/W4 /WX)
* Remove /wd4512 from CMakeLists.txt
* Suppress Visual Studio 4512 warning ('class' : assignment operator could not be generated)
