demonstrate using subtle-ng-derive with ConstantTimeCmp

[cosmicexplorer]

Problem

In #469 we decided to take a look at whether we could upstream some extensions to the subtle crate to support Ordering comparisons. After creating zkcrypto/subtle-ng#5, I created this PR to demonstrate how adopting subtle::ConstantTimeCmp from that change could improve the code.

Solution

• I created the subtle-ng-derive crate in order to derive constant-time comparisons for aggregate structs using the technique developed in reduce boilerplate implementing comparisons for user-defined types zkcrypto/subtle-ng#6.
• I pulled in a branch of subtle-ng which contained the changes from define and implement ConstantTime{Partial,}Ord traits zkcrypto/subtle-ng#5.

Result

It is now possible to #[derive(ConstEq, ConstOrd, ConstantTimeEq, ConstantTimeCmp)] to get constant-time Eq and Ord implementations for several structs, which allows us to remove utils.rs.

[jrose-signal]

I'm going to close this because it's an extra dependency for something quite subtle (🥁), but it's certainly clever!

[cosmicexplorer]

Note: subtle-ng is unmaintained, and the original subtle crate should be used. I have raised this use case with upstream subtle project at dalek-cryptography/subtle#100 and hope to integrate the derive macros into the upstream subtle crate itself.