Add PartialEq derive to PublicKey

[jack-michaud]

Adds PartialEq, Eq, and Hash to derive statement for PublicKey.
Derives from the PartialEq impl for MontgomeryPoint.

Seemed like this was an easy win so users of this crate can use the PartialEq derive macro on structs that use PublicKey!

[hdevalence]

This seems good -- I think that in addition to PartialEq there should also be derived Eq and also Hash. Eq is needed to use the == operator and there's no downside to deriving Hash. (I think the only other derivable traits are Ord and PartialOrd, and those should not be implemented because there's no ordering on public keys).

[jack-michaud]

Thanks @hdevalence, good call -- just added Eq and Hash

[hdevalence]

Thanks! I'll publish this as 1.1.