Fix blobstore compilation on macOS and MSVC

macOS:

    src/blobstore.cc:25:73: error: typename specifier refers to non-type member 'impl' in 'org::blobstore::BlobstoreClient'
    BlobstoreClient::BlobstoreClient() : impl(new typename BlobstoreClient::impl) {}
                                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
    cxxbridge/crate/demo/include/blobstore.h:20:25: note: referenced member 'impl' is declared here
      std::shared_ptr<impl> impl;
                            ^

MSVC:

    src/blobstore.cc(25): error C2061: syntax error: identifier 'impl'
diff --git a/demo/include/blobstore.h b/demo/include/blobstore.h
index d89583a..68a7fc2 100644
--- a/demo/include/blobstore.h
+++ b/demo/include/blobstore.h
@@ -16,8 +16,8 @@
   BlobMetadata metadata(uint64_t blobid) const;
 
 private:
-  class impl;
-  std::shared_ptr<impl> impl;
+  class Impl;
+  std::shared_ptr<Impl> impl;
 };
 
 std::unique_ptr<BlobstoreClient> new_blobstore_client();