Get rid of fallible-iterator as a direct dependency, import from postgres

[jacobsvante]

This is the only outdated dependency we have. The postgres crate exports fallible_iterator, which we could use.

[beanpuppy]

tokio-postgres doesn't appear to re-export it however: https://docs.rs/tokio-postgres/latest/tokio_postgres/#reexports. If we were to use the re-exported fallible_iterator for the sync postgres only we'd need to have two types of imports in the codegen, which I don't really think is worth doing.

[beanpuppy]

I also see both packages use fallible-iterator = "0.2" in their Cargo.toml. I suggest we follow suit if we want it to stay updated.

[jacobsvante]

Correct, the postgres crate publically exports fallible_iterator and tokio-postgres does not.

However, postgres is just a thin wrapper around tokio-postgres, so I'm thinking that maybe we can include both crates, always?

If we then always follow the fallible-iterator version that postgres exports, we don't have to worry about being out of sync.