`generic_associated_types` stabilized

[qm3ster]

The generic_associated_types is not only no longer marked incomplete but has in fact been stabilized and is no longer necessary to specify.
Please also consider updating the README accordingly.
Also, adding a return_position_impl_trait_in_trait mode, enabled via optional feature on this crate, would be absolutely hilarious.

[4lDO2]

I might look into it, but AFAICT it's only a matter of time before rustc itself finishes async_fn_in_traits. That said, PRs are welcome though.