PeerID Spec Conformance

[btoms20]

What:

• This PR brings swift-crypto into compliance with the Libp2p PeerID Spec.

Changes:

• When unmarshalling a ED25519 Private Key, we support the multiple formatting cases laid out in the spec
• When parsing a Secp256k1 Public Key, we now support the 5 different key formats
  • even, odd, uncompressed, even hybrid and odd hybrid
  • previously we only supported uncompressed 64 byte public keys
• Added support for compressing a Secp256k1 Public Key
• When marshalling a Secp256k1 Public Key we used the compressed (32 byte) version
• General code clean up

Tests:

• Added the test vectors outlined in the Spec