68c73850d8 Minimise FFI in the public API (Tobin C. Harding)
Pull request description:
Normal users should never need to directly interact with the FFI layer.
Audit and reduce the use of `ffi` types in the public API of various types. Leave only the implementation of `CPtr`, and document this clearly as not required by normal users. Done for:
- PublicKey
- XOnlyPublicKey
- KeyPair
- ecdsa::Signature
- ecdsa::RecoverableSignature
ACKs for top commit:
apoelstra:
ACK 68c73850d8
Tree-SHA512: 8242527837872f9aba2aab19b02c2280ca1eb1dfd33c8ca619726d981811d72de3e5a57cbde2fbe621eb8e50e43f488804cd51d27949459da1c0ceb03fca35e3