Add Redact to the Request and Response interfaces

[Richard Crowley]

This will give us a more structured Redactor than what's currently in Logger.

Also create an embedable NilRedactor to satisfy the interface for types that don't need any redaction.

[Richard Crowley]

Actually, instead of having to embed NilRedactor, we could redactor, ok := request.(Redactor) to keep the no-redactor-necessary case simple.

[Richard Crowley]

The Request and Response interfaces have been removed.