Various evidence-related fixes

[fjarri]

• Add Evidence to public exports and add docs.
• Fixes An invalid message evidence generated during an echo round cannot be verified #49
• Fixes MismatchedBroadcastsEvidence::verify should have its logic depend on error #52
• Fixes Better evidence description generation #51
• Don't require a verifier argument to Evidence::verify() - we already have it in the struct.