PublicKey type ignores validation on deserialization

[austinabell]

Noticed while doing something related, PublicKey just uses the borsh derive deserialize, which ignores validation and accepts any bytes.

Also, it doesn't implement hash, which was used by the multisig contract. Although it's generally better to avoid std hash collections in wasm contracts, probably worth it to have for tests. Thoughts @matklad if it's worth leaving out or adding Hash derive to public key?