derive Hash for RecoverableSignature

[NicolaLS]

It would be nice to also derive Hash for RecoverableSignature so data structures containing it don't have to implement it themself if they need to derive Hash

[apoelstra]

I'm gonna go ahead and merge this, since for most cases it won't matter at all, but strictly speaking we should impl this by calling the ffi _serialize method first, since different versions of the library (or the same version on different endianness) may have different encodings for the internal data of this type.

But because the output of std::Hash is basically never stored or exported in any fashion, only used as a key inside hash maps/sets, I'm happy to leave that for a future PR.

[apoelstra]

ACK e275166