This reduces per-scope overhead from minimally 6 words to 2 words, with one additional pointer per entry, rather than an average of 2 per entry for larger-than-4 element lists. For temp zone parsed functions it additionally makes the declaration-list actually freeable.

This introduces ThreadedList to implement the details of dealing with such a list.

BUG=v8:5209

Review-Url: https://codereview.chromium.org/2457393003
Cr-Commit-Position: refs/heads/master@{#40703}