add IntoFuture

[yoshuawuyts]

Adds the IntoFuture trait, wihch @Nemo157 pointed out would be useful for implementing the "async builder" pattern we used when building Surf.

In the RFC2394 async_await it's also stated that await should call IntoFuture under the hood.

A builtin called await! is added to the compiler. await! can be used to "pause" the computation of the future, yielding control back to the caller. await! takes any expression which implements IntoFuture, and evaluates to a value of the item type that that future has.

This PR doesn't add that functionality to await, but does have the right shape it should take, which allows people to start implementing IntoFuture so that later it will "just work".

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cc/ @withoutboats, is what I've said so far about IntoFuture still accurate? I didn't see any mentions that this had been changed in the stabilization report, so does that mean that it means this simply has not been implemented yet?

Screenshot

[ghost]

Do you think it would make sense to open this PR in the rust-lang/rust repository? :) It seems to me the intention was for this trait to exist in the standard library, but nobody has implemented it there yet.

[yoshuawuyts]

@stjepang yeah I think it does. I do think there's value in still merging the PR here though. I'll check with the lang team what they think and if they're okay with it I'll follow up with the PR to std.

[withoutboats]

@stjepang @yoshuawuyts I think IntoFuture (and making await work with that) would be a great extension, but probably one that would require a (small) RFC.

[yoshuawuyts]

There's now a PR to std for this also: rust-lang/rust#65244.