Revisit if we want Debug implementations on secret data

[coltfred]

We should decide if we want Debug implementations on secret data. Patrick was removing it as part of his memory protections PR, but I think it warrants a larger discussion.

We've discussed that we could conditionally remove the Debug as part of a feature flag, which would allow people who don't want to be able to use Debug in their codebases while still allowing it for others. This feature could be on by default if we wanted to be safe by default (which is always a good goal).

Even if we do remove it, logging the bytes is still possible with {:?}, prv_key.bytes().