This CL prepares JSON#stringify for improved error messages when
serializing circular structures. To this end, we also push the
key/index, in addition to the object itself, onto the stack that keeps
track of circular structures.

The stack itself is changed from a JSArray to a std::vector.

R=yangguo@chromium.org

Bug: v8:6513, v8:8698
Change-Id: I6dc4cb3be75a4514281411c654337f37c8798e55
Reviewed-on: https://chromium-review.googlesource.com/c/1424863Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59019}