Consider adding structured binding support to futures

[Dan Bonachea]

Split off from documentation issue #469:

The C++ standard specifies that structured bindings can use get<i>(e), found by ADL, to enable structured bindings for a custom class. By providing this function in the upcxx namespace for upcxx::future or as a hidden friend in upcxx::future, we can enable this functionality for UPC++ futures.

upcxx::future<int,double> fut = /* ... */;
auto [a,b] = fut;

[Dan Bonachea]

In the 2023-02-21 meeting discussion, we decided not to pursue this idea, which seems likely to degrade readability (e.g. result vs wait), and at best just saves a few characters of typing relative to calling the appropriate wait* or result* member function on the fut object (the best choice depends on the synchronization state and whether the caller wants the new variables a,b to be const references or copies).