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/thejerf/suture/pre-commit

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+# This ensures all executables build and all tests pass before a commit
+# goes through.
+
+set -v
+set -e
+
+CWD=`pwd`
+
+go test
+./gml .
+
+echo Build succeeds.
+exit 0
+
+