Summary:
[Folly] Introduce makePromiseContract, for producing a paired promise-and-semi-future or a paired promise-and-future, depending on which overload is invoked.

It is not ideal that a future is gotten from a promise, but only once. Rather, it is ideal that they be created together.

C++17 code may use structured bindings:
```lang=c++
auto [p, f] = makePromiseContract(); // promise with semi-future
auto [p, f] = makePromiseContract(&e); // promise with future, given executor e
```

Reviewed By: LeeHowes

Differential Revision: D7984050

fbshipit-source-id: 52ee5624fde7c48aa0e81243564ad067c6c6919f