Parsing of non-UTF8 CommitmentPrefix is incorrect

[Farhad-Shabani]

Bug Summary

When performing verify_(non_)membership in the Tendermint light client, we use apply_prefix to construct the merkle path necessary for proof existence checks. The apply_prefix converts the CommitmentPrefix to a string. However, if the prefix is not valid UTF-8, the function cannot handle it properly and outputs an incorrect string in the format of:
<not valid UTF8: {commitment_prefix:?}>

Proposal

• Construct the merkle path using bytes instead of strings (this will be addressed in imp: Enable custom paths for light client proof verifications #1273).

Version

=< 0.53.0

[Farhad-Shabani]

As for the Debug impl

We can't simply remove or alter the Debug implementation for CommitmentPrefix, as it is used as the type for the prefix field in connection messages. Therefore all the Debug impl up to the MsgEnvelope relies on having a Debug implementation for CommitmentPrefix.

Anyhow, since we plan to move away from the legacy apply_prefix and allow path bytes (#1273), we won't encounter the parsing issue in critical paths.

It also should be noted we can't return an error on the Err variant of Debug, as obtaining a string from messages containing non-UTF8 prefix bytes would result in an error. Therefore, the current Debug implementation looks fine.