`slog::Serializer::emit_serde` refers to crate feature `serde`

[larswirzenius]

https://github.com/slog-rs/slog/blob/master/src/lib.rs#L2728C1-L2729C30:

    /// To prevent pulling-in `serde` dependency, this is an extension behind a
    /// `serde` feature flag.

The feature seems to actually be called erased-serde