Code cleaning.
Added ctl::return_type helper.
Changed synthax of signal_t. Prepared ground for holdable slots.
Cosmetics.
Added template integer sequence generation.
Added result_of_functor and tuple_to_args.
Reworked signal/slot architecture: signal_t takes care of holding
connections and grouping calls, connection takes care of its lifetime
(stop()) and of making the actual call to the slot, slot simply calls
user code and never knows about the connection object.

Connections can thus now be unique (call the slot once and stop) and/or
holdable (keep arguments aside when blocked, and call slot on
unblock()) with minimal effort. Writting custom connections should be
easier now.