Expose `Buffer.completeSegmentByteCount` or equivalent

[JakeWharton]

As far as I can tell there's no way to replicate this functionality in the current public API.

Segment.owner isn't exposed, so a UnsafeBufferOperations.forEachSegment aggregation can't know if segments are safely full or not. This also comes into play if you've done a UnsafeBufferOperations.moveToTail of a non-standard size and it becomes unclear if Segment.remainingCapacity can be relied upon since it doesn't check owner flag.

I'm trying to do a compression loop where I only invoke the expensive compressor operation on completed segments (until flush/EOF).

[JakeWharton]

To be clear: I'd also be happy with some other API (even in unsafe) that let me accomplish this.

[fzhinkin]

it becomes unclear if Segment.remainingCapacity can be relied upon since it doesn't check owner flag

It has to be zero for a "shared" segment, as it's the only sane value for it.

And we can then provide something similar to completeSegmentByteCount on top of it