Restore flatbuffers::FlatBufferBuilder::kFileIdentifierLength, which was a documented part of the public API, but the identifier was lost during the refactoring effected by comment commit 6c8c291559.
* Added Google benchmarks (and gtests)
* Separate benchmark CMakeLists.txt to its own file
* Move output directory to target just flatbenchmark
* Reduced from encoding 210ns -> 188ns
* store size_ as uoffset_t
* fixed windows c4267 warning
* split flatbuffers.h into separate files
* wrong variable in cmakelists for android
* readded two accidentally deleted includes
* created buffer.h and moved buffer related things over
