* refactor: add a Torrent.id property
* feat: add Controller.torrentForId
* refactor: make tr_session_idle_limit_hit_func a std::function
* refactor: make tr_session_metadata_func a std::function
* refactor: make tr_session_ratio_limit_hit_func a std::function
* refactor: make tr_torrent_completeness_func a std::function
* refactor: make tr_altSpeedFunc a std::function
* refactor: make tr_sessionSetQueueStartCallback callback a std::function
* refactor: make tr_rpc_callback_status callback a std::function
* refactor: remove user_data arg from tr_torrent_rename_done_func
* refactor: harden `tr_torrentFindFromId()` by making it safe to pass in a nullptr session