all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/calmh/xdr/README.md

@@ -0,0 +1,18 @@
+# This project is not actively maintained
+
+Issues and pull requests on this repository may not be acted on in a timely
+manner, or at all.  You are of course welcome to use it anyway. You are even
+more welcome to fork it and maintain the results.
+
+![Unmaintained](https://nym.se/img/unmaintained.jpg)
+
+xdr
+===
+
+[![Build Status](https://img.shields.io/circleci/project/calmh/xdr.svg?style=flat-square)](https://circleci.com/gh/calmh/xdr)
+[![Coverage Status](https://img.shields.io/coveralls/calmh/xdr.svg?style=flat)](https://coveralls.io/r/calmh/xdr?branch=master)
+[![API Documentation](http://img.shields.io/badge/api-Godoc-blue.svg?style=flat)](http://godoc.org/github.com/calmh/xdr)
+[![MIT License](http://img.shields.io/badge/license-MIT-blue.svg?style=flat)](http://opensource.org/licenses/MIT)
+
+This is an XDR marshalling/unmarshalling library. It uses code generation and
+not reflection.