[secrets] Replace `Secret` specific traits with inherent methods

[robinhundt]

Currently, in secrets we have traits like IntOps which provide things like wrapping arithmetic for the Secret type.

When not using check-secret-independence, these traits are not necessary, as the corresponding methods on the public types are inherent methods. We could change these methods to also be inherent on the Secret type and thus remove the need for cfg gated trait imports. This would reduce the friction for adopting check-secret-independence in a crate that currently doesn't use it.