Remove boxing from futures example

[dimbleby]

The example in cares_futures.rs currently has queries returning BoxFuture. That's an unnecessary allocation best avoided by returning impl Future - if and when that stabilises.

See rust-lang/rust#34511.

[dimbleby]

Now using an unboxed structure, will go to impl Future when we get there... but I don't think I need this issue to remind me.

[ndusart]

@dimbleby impl Trait is stable now 🎉