This adds a CAPTCHA check during the registration flow. More precisely, the user now has to call the (new) method create_challenge that returns a CAPTCHA-style challenge (image and key) and register takes some extra parameters (the chars supposedly shown in the image, and the key). This involved some changes, in this repo and in others:

• Adapt the CAPTCHA library for it to use the IC's randomness: Allow setting Rng daniel-e/captcha#15,
• Update the png library used by the CAPTCHA library to build on wasm32-unknown-unknown: Add wasm32 implementation for libc kornelski/lodepng-rust#51,
• Added create_challenge : () -> (Challenge); with the corresponding "challenge" types:

type Challenge = record {
    png_base64: text;
    challenge_key: ChallengeKey;
};
type ChallengeKey = nat32;
type ChallengeResult = record {
    key : ChallengeKey;
    chars : text;
};

• ... as well as an extra argument to register: register : (DeviceData, ProofOfWork, ChallengeResult) -> (RegisterResponse);,
• A change of flow in the register frontend flow: the confirmation screen now shows a CAPTCHA,
• The backend-tests were updated to accommodate the new create_challenge() call.
• The rust build now has a dummy_captcha feature (enabled in selenium tests and backend-tests) which makes sure the CAPTCHA challenges expect a known value ("a"). This should of course NEVER be used in production, hence is opt-in.

This also led to some helpful, though not absolutely necessary changes for backend-tests:

• Fixed build failure after freezing cabal: Set lower bound on aeson ic-hs#58
• Update haskell-candid to (finally) accept our candid file: Ignore init arguments nomeata/haskell-candid#17

TODOs:

• Add backend-tests for CAPTCHA failure and "too many challenges inflight" Add backend tests for CAPTCHA #468
• Add selenium tests for CAPTCHA retry (i.e. auth device was created but CAPTCHA challenge failed -> only the CAPTCHA is reloaded)
• Add PoW to create_challenge
• Differentiate a "bad captcha" error from an actual canister, unexpected error.
• Make the challenge key a string