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/d4l3k/messagediff/CHANGELOG.md

@@ -0,0 +1,14 @@
+## nightly
+* Added support for ignoring fields.
+
+## v1.1.0
+
+* Added support for recursive data structures.
+* Fixed bug with embedded fixed length arrays in structs.
+* Added `example/` directory.
+* Minor test bug fixes for future go versions.
+* Added change log.
+
+## v1.0.0
+
+Initial tagged release release.