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:

• https://github.com/ZcashFoundation/zebra/pull/5960
• https://github.com/ZcashFoundation/zebra/pull/5960/commits/910f9b791a32ffbc588fe07aac42059a058cee76