Recent Rust releases require try_trait_v2 not try_trait

[cameronelliott]

The documentation reports that #![feature(generators, generator_trait, try_trait)] is required.

In recent Rust releases, try_trait has been removed, and try_trait_v2 has been added.

[withoutboats]

I no longer maintain this library, but if you make a PR I will merge it and cut a release.