flatbuffers: 1.4.0 -> 1.8.0

Semi-automatic update. These checks were performed:

- built on NixOS
- ran `/nix/store/2zfi5j6wmfb6hbaqmmsbarbjz8r5f333-flatbuffers-1.8.0/bin/flatc --version` and found version 1.8.0
- found 1.8.0 with grep in /nix/store/2zfi5j6wmfb6hbaqmmsbarbjz8r5f333-flatbuffers-1.8.0
- found 1.8.0 in filename of file in /nix/store/2zfi5j6wmfb6hbaqmmsbarbjz8r5f333-flatbuffers-1.8.0