Rename feature 'ser_de' as 'serde'
When I introduced the 'ser_de' feature, I couldn't find a way to name it 'serde' while also making it pull the optional 'serde' crate together with the other related dependencies.
With rustc >= 1.60 we can use 'dep:serde' to refer to the 'serde' dependency as part of the 'serde' feature.