Up until now, we've mostly executed futures by using .await, which blocks
the current task until a particular Future completes. However, real
asynchronous applications often need to execute several different
operations concurrently.
In this chapter, we'll cover some ways to execute multiple asynchronous operations at the same time:
join!: waits for futures to all completeselect!: waits for one of several futures to complete- Spawning: creates a top-level task which ambiently runs a future to completion
FuturesUnordered: a group of futures which yields the result of each subfuture