Add Iterable bitset class. This class allows us to quickly notify Impl/Renderer layers of which state changed. BUG=angleproject:1040 TEST=angle_unittests (Mac/Win/Linux) Change-Id: I2f27bc65b4f627a261e94e3e3b7095b169986cc6 Reviewed-on: https://chromium-review.googlesource.com/289557 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>