Seal KeyPair type

[yoshuawuyts]

We're forwarding the KeyPair type from another crate. We should create a wrapper type for this.

We should also be able to have a half-opened state for these - e.g. read-write vs just read

[soyuka]

We should also be able to have a half-opened state for these - e.g. read-write vs just read

Could the PartialKeyPair I added in #18 be the answer?

[yoshuawuyts]

@soyuka yeah, definitely! Ideally we could remove any dalek structs from our public interface, and replace it the partial keypair. Would you like to give that a shot?

[soyuka]

We're not forwarding Keypair anymore if I'm not mistaken. The only thing I see here are Feed and FeedBuilder having PublicKey and Option<SecretKey>. Those two should stay like this.

You can close this imo.