unsafe optimizations

[Kixunil]

I wonder if it'd be useful to have a feature unsafe-optimizations that turns on unsafe code with the only intention to optimize. Currently the candidate is using [MaybeUninit<u8>; 65] inside SerializedPublicKey. Relies on correct impl in secp256k1 ofc but that one already calls memset on the whole array 🙈

[dr-orlovsky]

I am not against unsafe when it is behind the feature