We are ready to release a new minor version of `secp256k1-sys`, in order
to do so we must make change the symbol names to reflect the new version
as well as the usual changelog and version bump.
In preparation for releasing `secp256k1-sys` v0.7.0 do:
- Rename symbols to from `0_6_1` -> `0_7_0`, done mechanically (search
and replace)
- Add changes log notes
- Bump `secp256k1-sys` crate version 0.6.1 -> 0.7.0, justified because
we have added new public methods to various types (e.g.,
`PublicKey::cmp_fast_unstable`)