Implement fastrlp traits for Signature

[Rjected]

Motivation

Deriving fastrlp Encodable and Decodable traits for types that contain Signature would not be possible without implementing Encodable and Decodable for Signature.

Solution

• Add fastrlp as a dependency to ethers-core, meaning this conflicts with Implement fastrlp traits for Bytes #1443 for the time being
• Implement Hash on Signature (just a convenience impl, not related to fastrlp traits)
• Implement RLP encoding and decoding for Signature. This does not encode or decode a RLP header for a signature type, since types that would contain a signature (for example a signed transaction) are usually encoded as [...rest of object, v, r, s] rather than [...rest of object, [v, r, s]].

PR Checklist

• Added Tests
• Added Documentation
• Updated the changelog