Method release

Bcore/src/main/cpp/Dobby/tests/catch.hpp:9434–9437

  }

  void release(std::size_t index) {
    m_streams[index]->copyfmt(m_referenceStream); // Restore initial flags and other state
    m_unused.push_back(index);
  }

  // !TBD: put in TLS
  static auto instance() -> StringStreams & {