Don't serialize redacted headers

[hadley]

This required quite a lot of refactoring to clarify what's actually going on. It should now be clearer that there is a redacted sentinel value which is used to nicely format, print, and str redacted values that might be exposed to the user (I don't fully recall why I added this but I suspect it's because we only want to apply cli formatting at the exact time when the value is displayed). This code now lives in utils-redacted.R.

I also clarified the type of the headers object — the componets can either be an atomic vector or a weakref. And since redacted components now have their own type, we no longer need the redact attribute.

Fixes #721

[shikokuchuo]

Practically there's no way to access the 'value' from the 'key' if the weakref itself is no longer around, hence it's probably fine that you tie the lifetime of 'value' to the package the environment.

However, if you want to ensure the lifetime of 'value' is really tied to a single request, then you could instead pass the request req as an argument to as_header() and consuming functions, and you'd then be able to use req$state as the 'key'.

[hadley]

I decided to make the lifespan a bit more explicit and I do think it makes it clearer.