Implement Pedersen hash

[xJonathanLEI]

We need to implement the equivalent of pedersen_hash from cairo-lang as we'll need it for the signer.

[xJonathanLEI]

As I'm totally not a cryptographer, the easiest solution to me is to simply create Rust bindings for StarkWare's crypto-cpp library, which already contains the functionalities we need. Tracked here.

[xJonathanLEI]

The wrapper is now done at starkware-crypto-rs and published to crates.io. It's technically ready for use.

However, it's discovered that a serious performance issue (previously reported here) exists in the library: calling Sign() is very slow. It took over 7 seconds to do a single Sign() call on my machine.

Apparently this is unusable. We need to find an alternative solution.

[xJonathanLEI]

The performance issue seems to be related to C++ optimization levels, and was fixed in starkware-crypto-rs v0.1.1.

Signing time under the new version is around 180ms on my machine. It's still quite slow, but I think we can deal with it until either a better alternative is available, or we implement it in Rust.