Serde forces std in 2.0.0-pre.0

[semenov-vladyslav]

Enabling serde feature only (without std feature) enables use of std crate which shouldn't happen. It seems due to bad dependencies in ed25519-dalek-2.0.0-pre.0.

[robjtede]

Looks like serde_bytes has their std feature as default, and it's not disabled for the serde feature of this crate.