Using futures

A future represents a value that will be available later, or an error, similar to the Result type. A stream represents multiple values (or errors) that will be available at different times in the future, similar to an Iterator<Result<T>>. This crate provides many combinators such as and_then(), map(), and others similar to the one available on the Result type. But, we won't use them, preferring the async/await syntax that we'll see later.