This switches the {StdlibSet} to be a data structure that does not
require dynamic memory allocation. This makes it easier to carry it
around as part of a {CompilationJob} and serialize it into the heap.

R=clemensh@chromium.org

Change-Id: I77b2353cfdcd9438a26f04d00749159fed9b9b6c
Reviewed-on: https://chromium-review.googlesource.com/584868
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46889}