2.99.6 (unstable):

* signal: Remove slots() and the container that it returns,
  and the iterator for that container.
  See https://mail.gnome.org/archives/libsigc-list/2016-April/msg00009.html
* signal: connect(): Return a connection, instead of an iterator that
  can be implicitly converted to a connection.
* Use std::shared_ptr<> for signal_impl reference-counting.
  Instead of manual reference-counting.
  (Murray Cumming) Bug #764935
* slot: Use unique_ptr<> to avoid direct call of destructor.
  (Marcin Kolny) Bug #764935
* Add optional benchmark code, using boost::timer.
  (Murray Cumming)
* Make some single-item constructors explicit.
  (Murray Cumming)
* Make some member variables and methods private.
  (Murray Cumming)
* Remove _ and __ prefixes on variables and parameters.
  (Murray Cumming)
* Header guards: Remove leading underscores (and trailing underscores).
  (Murray Cumming)
* limit_derived_target<T_Target>: Rename to limit_trackable_target<>
  and visit_each_type<T_Type>() to visit_each_trackable() to make their purpose
  more obvious.
  (Murray Cumming)