ScretKey::sing & Signature::recover returned error

[elpiel]

Since https://docs.rs/ethsign/latest/ethsign/struct.SecretKey.html#method.sign returns the secp256k1 can we consider exporting the error or just use the ethsign::Error instead?

It would be nice to have SecretKey::sing directly use an array [u8; 32] instead of slice &[u8]. This would improve the api since the message should always be 32 bytes.