Implement Debug for async futures/streams

[jf2048]

Just for completeness, these were the only things I noticed where it was missing.

[zesterer]

Thanks!

Edit: It seems like this PR is failing, perhaps it's better to implement Debug manually? Avoiding revealing internal implementation details would also be good, I think.

[jf2048]

Whoops, I guess my rust-analyzer had a hiccup or something :)

I changed it to have Debug on those private structs, but I also can change it to just be an empty implementation of Debug for those fields/variants, would that work?

[jf2048]

Now changed it to have empty implementation

[zesterer]

Thanks, this is ideal. It seems there is a conflict though, but this should be a trivial change.