Extract public keys comparison to a helper

[adambabik]

In a few places, we rely on the checking equality of public keys. We use elliptic curves which means we have three parameters to compare: type, X and Y coordinates.

Currently, in a few places this can be found:

if publicKey.X.Cmp(c.PublicKey.X) == 0 && publicKey.Y.Cmp(c.PublicKey.Y) == 0 {
// ...
}