A new type for data that should never be logged.

[Eh2406]

Cargo occasionally deals with secrets. We should have a new type that does not implement Display that is used in the type system to mark "this value is secret and should not be printed". Types that should be converted are plaintext tokens and PASETO after they've been generated.

cc:

• Asymmetric tokens #10771 (comment)
• Tracking Issue for RFC 3231: Asymmetric Tokens #10519

[kylematsuda]

@rustbot claim