Summary:
`collectAll` allows to `co_await`s multiple tasks using structured concurrency.

Unfortunately `future::collectAny` does not follow the structured concurrency pattern, and detaches the uncompleted operations.
This can result in memory errors (the coroutines access data that has already been freed).

This diff introduces `coro::collectAny`, which given a number of awaitables it returns the result of the first awaitable to finish, in addition to its index, cancels the remaining operations **and waits for them to complete**.

The implementation uses `collectAll` as a building block.
The return signature mirrors the one from `future::collectAny`.

Reviewed By: yfeldblum, rptynan

Differential Revision: D28945040

fbshipit-source-id: 402be03e004d373cbc74821ae8282b1aaf621b2d