Fix interledger-packet::ErrorCode too lenient parsing

[koivunej]

Found this crash stashed away, which wasn't because of ErrorCode or parsing, but because of too strict fuzz_target. This PR fixes:

• allow only IA5String (ascii) error codes as is in the ASN.1 definition
• handle debug and display output for the error codes with control characters gracefully
• remove string allocation in ErrorCode's Debug impl

There's some churn related to the roundtrip test fn being added and removed, but I'll follow up with a better fuzz/fuzz_targets/packet.rs after this PR.

Cc: #705

[koivunej]

Oki, a few force pushes later, I think this is ready to go :)