Add the feature "serde" to the crate either

[masklinn]

The playground is nice to demo various things including serde. either has serde support, but both the basic externally tagged support and the opt-in untagged support are gated behind a "serde" feature: https://github.com/rayon-rs/either/blob/main/Cargo.toml#L21 so that can't be used on the playground without "vendoring" the crate (at which point having it seems a shame)

[shepmaster]

You'll need to ask the either maintainers to enable that in their Cargo.toml.