Provide `to_sec1_bytes` for `PublicKey`

[KizzyCode]

Is there a reason why there is no to_sec1_bytes-method?

To parse a SEC 1 representation, we have PublicKey::from_sec1_bytes – however to serialize into a SEC 1 representation we need to detour via EncodedPoint::from(&public_key) and then use encoded_point.as_bytes(). Once you know this, it is pretty trivial – however it took me a few minutes to get there and its not that obvious IMO.

[tarcieri]

A PR to add such a method would be accepted.

It would need to be gated on the alloc feature, and could call EncodedPoint::to_bytes.