gzip-iso: bugfix in zstate setup, cosmetic refactoring
gzip-iso: Speedup some cases by using more memory.

Significant speedup on some cases by using roughly another index size
in ram. The ram usage is now up to roughly cache size plus 2x index size.

This patch adds another index-like direct access point for each span we've
visited. This replaces the single z-state which was used for
sequential extraction, and does the same, but now it can continue
sequentially on most previously visited spans instead of only from
the last read.