Implemented branching and merging of Environments to facilitate handling of
conditional and unconditional jumps in the SerializerForBackgroundCompilation.
Added tests and printing helpers for the Environment. The internal structure
of the hints was changed to ZoneSet to support avoiding of duplicates.
Alternative implementation considerations were documented here:
https://docs.google.com/document/d/1vCQYhtFPqXafSMweSnGD8l0TKEIB6cPV5UGMHJtpy8k/edit?ts=5bf7d341#heading=h.jx4br0df5qzm

R=neis@chromium.org

Bug: v8:7790
Change-Id: Ib929c75ddb7f7fb290a5ca28d4422680a1514a4f
Reviewed-on: https://chromium-review.googlesource.com/c/1451847Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59534}