This CL gives a first look at the new way to represent tagged object
pointers in C++.
It adds infrastructure in Handles and the garbage collector to deal
with the new object type, and ports a first class to the new world.

Design overview: https://goo.gl/Ph4CGz

Bug: v8:3770
Change-Id: I3e37fbf399612f95540cb386710a595069fb9d55
Reviewed-on: https://chromium-review.googlesource.com/c/1292673Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56964}