Serde support for k256::schnorr::VerifyingKey

[ycscaly]

Whilst there is serde support for the ecdsa verifying key, and for the underlying curve point & scalar - the schnorr structs do not implement it. They do implement a to_bytes() but I believe it is helpful to keep a unified interface.

[tarcieri]

Yes, this would be good to add

[ycscaly]

Would you like me to do a PR, or do it internally?

[tarcieri]

A PR would be good