Add Future::join and Future::try_join

[yoshuawuyts]

Similar to #405's addition of Future::race we should expose Future::join and Future::try_join as well. However unlike #405 we would keep the macro around because it's the only way of creating a variable-sized tuple when joining multiple results ("return-type polymorphism").

This would be a chance as well to update the future module-level docs to talk about Future::join vs Future::race, making it more symmetrical.

This should be a fun challenge to pick up for whoever is interested. Marking it as "good first issue". Thanks!

[richard-uk1]

I think this issue has been resolved?

[yoshuawuyts]

@derekdreery indeed it is. Thanks for pointing it out!