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/generate.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+go run cmd/genxdr/main.go -- bench_test.go > bench_xdr_test.go
+go run cmd/genxdr/main.go -- encdec_test.go > encdec_xdr_test.go