The past re-factoring inadvertently increased memory consumption for
AstConsString. This implements a micro-optimization to revert and slightly
improve beyond the original state.

Example, Zone size for parsing closure.js:
  - 20,999,848 B (before refactoring)
  - 21,651,056 B (after refactoring patch; 3.1% regression)
  - 20,641,320 B (after this CL; 1.7% improvement over original)

(Reason: ZoneLinkedList requires 4 pointers to support
the std::list functionality (Zone*, head/tail ptr, payload ptr).
But since we only append and iterate in order and have the Zone*
available in the context, a super simple linked list (value + next ptr)
saves a bit of memory, especially for the common case of having 0 or 1
string segments.)

BUG=v8:6902, chromium:706935

Review-Url: https://codereview.chromium.org/2792353002
Cr-Commit-Position: refs/heads/master@{#44385}