Consider making "event_source::operator<<" transaction wrapper

[YarikTH]

Description

Currently operator<< is just a fancy way on writing src.emit(...);

Following code just emits 3 events in separate propagation turns.

ureact::event_source src = make_source<int>();
src << 1 << 2 << 3;

But it can be changed to do so atomically in a single turn. To do so "event_source::operator<<" should return some RAII proxy that starts transaction in the constructor, finishes it in the destructor and provides it's own operator<< to emit further events.

It would be a breaking change and I'm not sure if it is needed at all. But at the first glance it looks like a neat feature.

[YarikTH]

There is additional place where it can be considered - event_source::begin()

ureact::event_source src = make_source<int>();
std::copy( v.begin(), v.end(), src.begin() );

Currently it emits each event in a separate turn, but such iterator can as well perform transaction. Unlike operator<< this approach doesn't have reserve syntax to send events separately, so other method and iterator might be needed.