Trylle

#699: add a go.mod module file

#701: stop generating package "// import" comment

#741: deprecate {Unm,M}arshalMessageSet{JSON}

#760: different internal implementation of oneofs

.pb.go files generated by protoc-gen-go@v1.3.0 will require the proto@v1.3.0 package to work

various minor changes to code generation