This is preparation to support the Torque port of Object.fromEntries,
including tests to make sure that the interface of the iterator functions
is correct and compiles when used.

Change-Id: I2a30ef80a80f42d4744a92746c8cd383abc10c19
Reviewed-on: https://chromium-review.googlesource.com/c/1303726
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57192}