Please consider making rsyncable non-experimental

[joshtriplett]

Is your feature request related to a problem? Please describe. I'd like to make use of zstd rsyncable mode, without having to rely on experimental library features.

Describe the solution you'd like I'd love to have zstd rsyncable mode marked non-experimental (assuming it is, in fact, non-experimental).

(https://github.com/facebook/zstd/issues/3153 may need to get addressed first.)