so-5.5.1: New overloads for agent subscription methods

There are two new event methods for subscription of an event to a signal.

The first one related to event subscription for ordinary agents. It is possible to subscribe event that way:

so_subscribe_self().event< msg_get_status >( &my_agent::evt_get_status );

instead of:

so_subscribe_self().event( so_5::signal< msg_get_status >, &my_agent::evt_get_status );

The second one related to ad-hoc agents:

coop->define_agent().event< msg_get_status >( mbox, [] { return "ready"; } );

instead of:

coop->define_agent().event( mbox, so_5::signal< msg_get_status >, [] { return "ready"; } );

The old form with so_5::signal<MSG> marker is still supported and will be supported in the future versions of SObjectizer. Moreover the so_5::signal<MSG> marker is necessary for event unsubscription:

// Event subscription.
// New way...
so_subscribe_self().event< msg_get_status >( &my_agent::evt_get_status );
// Old way (verbose, but still pretty usable)...
so_subscribe_self().event( so_5::signal< msg_shutdown >, &my_agent::evt_shutdown );
 
// Event unsubscription.
// Can be done only via so_5::signal marker.
so_drop_subscription( so_direct_mbox(), so_5::signal< msg_get_status > );
so_drop_subscription( so_direct_mbox(), so_5::signal< msg_shutdown > );