This turns the ZoneList with minimum 6 words overhead into a linked list through variables, using 2 words for the empty list. Additionally the average number of pointers per entry goes down to the optimal 1 per variable that's in a list.

This does introduce 1 pointer unnecessary overhead for dynamic variables. If that becomes a problem we could distinguish between variables in lists and variables not in lists. We can distinguish them at construction-time.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2475433002
Cr-Commit-Position: refs/heads/master@{#40714}