Major refactor / rewrite

[Sajjon]

This is a big PR with a major refactor / rewrite of the impl which also introduces some breaking API changes.

First of all, the K1.PrivateKey / K1.PublicKey key pair has been removed, in favour of more CryptoKit like APIs splitting the keys up in multiple key pairs:
K1.ECDSA.Recoverable.PrivateKey / K1.ECDSA.Recoverable.PublicKey
K1.ECDSA.NonRecoverable.PrivateKey / K1.ECDSA.NonRecoverable.PublicKey
K1.Schnorr.PrivateKey / K1.Schnorr.PublicKey
K1.KeyAgreement.PrivateKey / K1.KeyAgreement.PublicKey

The ECDSA signing methods now use this format:
signature:for:options and signature:forUnhashed:options

The Schnorr validation methods now use this format:
isValidSignature:hashed and isValidSignature:digest andisValidSignature:unhashed

The ECDSA validation methods (shared for Recoverable and NonRecoverable) now use this format:
isValidSignature:hashed:options and isValidSignature:digest:options andisValidSignature:unhashed:options

Furthermore more Serialization and Deserialization methods have been added to:

• (all) PrivateKey(s)
• (all) PublicKey(s)
• (all) ECDSASignature(s)
• Schnorr.Signature

Such as pem, der and more.