Format `Signature` and other types as hex in Debug implementation

[teor2345]

Currently redjubjub formats Signature as an array of integers, which makes signatures hard to debug:

[39, 102, 7, 25, ... ]

Most cryptographic libraries format their types as a hexadecimal string.

This impacts the following types:

• Signature
• VerificationKeyBytes

You can see an example of these changes for redpallas in:

• fix(diagnostics): Hex-encode debug format of commitments, nonces, and nullifiers zebra#5960
• ZcashFoundation/zebra@910f9b7

[conradoplg]

Transferred from redjubjub, but haven't checked yet if it still applies