Similar in spirit and implementation to how `serde` supports borrowed
deserialization lifetimes. Like `serde`, it uses `'de` as the lifetime
of the trait (`FromoSql<'de>`) and it adds all borrowed liftimes as
lifetime bounds on `'de`.

Also like `serde` all top level references automatically have their
lifetimes borrowed, but container types carrying references must
explicity be borrowed with the `#[postgres(borrow)]` annotation.