so-5.5.8: Simpification of ad-hoc agents subscription

Since v.5.5.8 a reference to ad-hoc agent proxy can be used for subscription to ad-hoc agent's direct mbox. For example:

// v.5.5.8 code:
env.introduce_coop( []( so_5::rt::agent_coop_t & coop ) {
  auto a = coop.define_agent();
  a.event< some_signal >( a, [] {
    ... // Some event processing code.
  } );
  a.event( a, []( const some_message & msg ) {
    ... // Some event processing code.
  } );
  ...
} );

// Pre v.5.5.8 code:
env.introduce_coop( []( so_5::rt::agent_coop_t & coop ) {
  auto a = coop.define_agent();
  a.event< some_signal >( a.direct_mbox(), [] {
    ... // Some event processing code.
  } );
  a.event( a.direct_mbox(), []( const some_message & msg ) {
    ... // Some event processing code.
  } );
  ...
} );