Add CXX tutorial (#1392)

Add a number of slides that cover most of CXX's functionality and
demonstrate how it can be used.

Fixes #823.

---------

Co-authored-by: Martin Geisler <mgeisler@google.com>

third_party/cxx/blobstore/include/blobstore.h

@@ -0,0 +1,32 @@
+#pragma once
+#include "rust/cxx.h"
+#include <memory>
+#include <set>
+#include <unordered_map>
+#include <string>
+
+namespace org {
+namespace blobstore {
+
+struct MultiBuf;
+struct BlobMetadata;
+
+class BlobstoreClient {
+public:
+  BlobstoreClient();
+  uint64_t put(MultiBuf &buf);
+  void tag(uint64_t blobid, rust::Str tag);
+  BlobMetadata metadata(uint64_t blobid) const;
+
+private:
+  using Blob = struct {
+    std::string data;
+    std::set<std::string> tags;
+  };
+  std::unordered_map<uint64_t, Blob> blobs;
+};
+
+std::unique_ptr<BlobstoreClient> new_blobstore_client();
+
+} // namespace blobstore
+} // namespace org