Imports of package prekey

PathSynopsis
go.mau.fi/libsignal/eccPackage ecc provides a way to generate, sign, and use Elliptic-Curve X25519 Cryptography keys.
go.mau.fi/libsignal/keys/identityPackage identity provides identity keys used for verifying the identity of a signal user.
go.mau.fi/libsignal/util/optional