Copyright 2008 Google Inc.
https://developers.google.com/protocol-buffers/
It's a fork of popular serializer from https://github.com/protocolbuffers/protobuf fixed to work with Unity due to:
protocolbuffers#8476 protocolbuffers#8675
NuGet package: https://www.nuget.org/packages/Neon.Protobuf-Unity