Try haybale-pitchfork to detect constant time violations

[LLFourn]

See: https://github.com/rust-bitcoin/rust-secp256k1/pull/213

Obviously all the functions in https://github.com/LLFourn/secp256kfun/blob/master/secp256kfun/src/backend/parity/constant_time.rs should be constant time and we should be able to empirically verify they are and do the tests in CI. There are several obvious violations here but as we fix them they should be tested.

Thanks to @thomaseizinger for putting me onto this.