Extend hpack mod visibility in 'unstable' feature set

[cfircohen]

h2 users may find the hpack module useful as an independent module. Kindly extend its visibility for users who accept the risk of using the "unstable" feature flag.

This is implemented in #824

Thanks and happy new year!

[seanmonstar]

Would you be able to provide more of a motivation or use case? I'm certain users could use it in theory, but I'm hesitant to increase the maintenance burden for hypotheticals. :)

[cfircohen]

Sure, one use case is non-async code that parses h2 frames directly using frame and not codec. In this case, we'd need access to hpack too.
Thanks for the follow up!