rename `serde1` feature to `serde`

[lcnr]

The serde feature is currently called serde1.

This because serde derive requires that the crate serde is imported as serde and features share the same namespace with dependencies.

As I need to explicitly enable glutin/serde I currently use serde1 as the feature flag and emit a compilation error in case serde is used without serde1.

[lcnr]

for reference: https://github.com/serde-rs/serde/issues/953