Rename stream_extend to extend

[ghost]

I've renamed this method to be more consistent with the standard library and re-exported the Extend trait in our prelude.

[ghost]

@yoshuawuyts How do you feel about these ideas (all of them are independent of each other)?

• We don't put our Extend into the prelude so that synchronous foo.extend(bar); still works.

• In examples, istead of doing stream::Extend::extend(&mut foo, bar).await, first import with use async_std::stream::Extend::extend and then do extend(&mut foo, bar).await.

• Or, perhaps even re-export a convenience function called extend into the stream module so that one can do stream::extend(&mut foo, bar).await. I kind of like that - while the std library doesn't have iter::extend because it's not needed, this reminds me of io::timeout where we can add an extra function for convenience while still keeping all APIs std already has.

We discussed this already, but am curious about another round of opinions? Simple yes/no answers will suffice :)

[yoshuawuyts]

@stjepang okay with adding a new free extend method that calls the trait. I think that would solve most of the UX issues. Ok with not including it in the prelude, though still think there's a case to be made I don't feel strongly enough to push for it.

Patch seems good overall!

[ghost]

Here's an idea: what if both std::iter::extend() and async_std::stream::extend() existed?

That'd be nice and would make sync/async versions convenient and symmetrical at the same time :)

Let's discuss this offline one more time (briefly) and make a final decision. Thanks for clarifying motivation for this change last time we discussed it, that was helpful!

[yoshuawuyts]

LGTM!