I wrote this blog post that describes the library in more detail:
https://bluejekyll.github.io/blog/rust/2018/12/27/announcing-pg-extend.html
a link to the repo:
https://github.com/bluejekyll/pg-extend-rs
There's a bunch more work todo before this will be super useful, it only allows for one type conversion from Postgres to Rust ATM, but I hope to make it better.
If you use it, and have any feedback, I'd love to hear it, thanks!