add standard code generation header to vm/helpers.go

Author: Tim Cooper

vm/generate/main.go

@@ -19,6 +19,8 @@ func main() {
 		data += fmt.Sprintf(s, xs...) + "\n"
 	}
 
+	echo(`// Code generated by vm/generate/main.go. DO NOT EDIT.`)
+	echo(``)
 	echo(`package vm`)
 	echo(`import (`)
 	echo(`"fmt"`)