message: Add getter for the byte buffer
diff: @@ -63,6 +63,16 @@ public:
  error apply_raw(size_t num_bytes, void* data) override;
  + // -- properties -------------------------------------------------------------
  +
  + buffer& buf() {
  + return buf_;
  + }
  +
  + const buffer& buf() const {
  + return buf_;
  + }
  +
  protected:
  error apply_impl(int8_t&) override;
  