expose 2-key version of PublicKey::combine

Make a bunch of dependencies optional, increase major version number

Implement serde traits for Signature

Update serde to 1.0

add compact signature encoding

Use safe copy_from_slice() rather than unsafe copy_nonoverlapping()

Implement From<[u8;32]> for Message

Improve Error::InvalidMessage comment

Use DefaultHasher in place of the deprecated SipHasher

Annoyingly, this is a major version bump since we expose libc types in the FFI module

Turns out you cannot initialize constant SecretKeys in any way; these
two constants should cover most sane use cases (other good choices
are the SECG generator and the Alpha CT generator, but these will
wait for a major CT-supporting upgrade, unless demand for them appears.)