Feature request: allow to return value from C++ proxying API

[RReverser]

It would be nice to be able to return custom value directly from queue.proxySync(...) / proxySyncWithCtx / proxyCallback's callback high-level APIs.

Most proxying operations require returning a value, but right now the closest workaround is to create a value with default constructor and override it from the callback, which is a bit cumbersome and also not ideal when default constructor is potentially expensive. Another alternative is to use std::optional + emplace, which is less expensive, but even more cumbersome.

I'm not sure how that would play with bool - I guess bool should stay as the return type for proxySync<void> and then std::optional<T> would be the return type for proxySync<T>? Personally I almost always need to hard assert on proxying being successful, so I'd also be happy with something like T proxySyncChecked<T>() which would just throw exception / abort on proxying failure and return value directly otherwise.

[RReverser]

Related thought: the C++ API could probably integrate with std::promise. It already has the right interface to set value & exception separately, as well as exposes std::future to actually wait for the result and get values back, and it's all already part of the standard library so would probably be more familiar to consumers than a custom API for returning values.

So there would be std::future<T> queue.proxyFuture(...) than can either take a callback with no arguments - in which case the return value of the callback is propagated back to the caller, like in proxySync - or a callback with a std::promise argument, in which case it's the responsibility of the callback to do promise.set_value / promise.set_exception, similarly to proxyAsync.