Update `futures-macro-async` proc-macro2 usage from 0.2 to 0.3

[peterdelevoryas]

Currently, I can't seem to get #[async] to be found with features = ["nightly"], and cd futures-macro-async; cargo build --features nightly shows that the crate is currently broken from the new proc-macro api. I actually made a PR to futures-await doing this update, not realizing that I probably should have been making the changes to this repo, because futures-await is deprecated now. (I think? Maybe not though?) Anyways, I'm still not really sure why I'm getting the "not found" error, which is another issue that probably needs to be created.

Edit: Oh I see now why it's not being found!

[peterdelevoryas]

Oh woops, maybe I'm out of the loop, because it looks like #940 has all the same changes as my futures-await PR? This issue can be closed if this is all known/being reviewed/etc!

[hcpl]

AFAIK https://github.com/alexcrichton/futures-await is for tracking futures-await 0.1 (evidence from alexcrichton/futures-await#84 (comment)); and this repo contains everything for 0.2 release. I'm sure from now on most would want to contribute to this one, though seeing this PR: alexcrichton/futures-await#86 makes me think that futures-await 0.1 branch will still be supported for some time.

And just to clear more confusion around repo/crate names, instead of a future-await crate that depends on futures-await-async-macro and futures-await-await-macro, the async/await functionality is now covered by futures-async-runtime, futures-macro-async and futures-macro-await that should be available directly from futures 0.2 with "nightly" features enabled (which haven't been published yet, as you found out from this commit).

[peterdelevoryas]

Fixed by #940, closing!