Revert of Replace PushIfAbsent by a Stack object and move StringBuilderJoin to JS (patchset #6 id:100001 of https://codereview.chromium.org/1775403008/ )

Reason for revert:
[Sheriff] This lets a gc stress test time out:
https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20gc%20stress/builds/2337/steps/Mjsunit%20%28flakes%29/logs/regress-542823

The test ran in <2sec before this. Please fix the test as well on a reland.

Original issue's description:
> Replace PushIfAbsent by a Stack object and move StringBuilderJoin to JS
>
> This significantly speeds up String(array).
> BUG=
>
> Committed: https://crrev.com/c91faa0b39b62025460eb9f8b578e20d88f3549e
> Cr-Commit-Position: refs/heads/master@{#34696}

TBR=cbruni@chromium.org,adamk@chromium.org,bmeurer@chromium.org,verwaest@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review URL: https://codereview.chromium.org/1785003004

Cr-Commit-Position: refs/heads/master@{#34706}