I decided to make the new method pure-virtual as I wanted to make sure that all wrapper streams properly delegate to the inner stream. We wouldn't want e.g. proactive HTTP cancellation to unexpectedly not work when running over TLS.

Move generic gzip output operations to private helper for easier sharing of capabilities between GzipOutputStream and GzipAsyncOutputStream.

 - Makes it possible to decompress data granularly via push-based API.
 - Adds ability to flush output at an arbitrary point.
 - Removes manual `ended` check as zlib checks it for us.

@kloepper pointed out a while back that every compiler you've ever heard of supports this. Plus, it's more concise, it's not prone to copy-paste errors, and it looks nicer.

At the time I wanted to remain consistent and I didn't feel like spending the time to update all my existing code. But, every time I've added a new header since I've cursed the include guard, so I finally broke down and changed it.