This extracts the parts of the TypedSlotSet that are used only
sequentially into a separate class called TypedSlots.

The new class will be used in the concurrent marker to keep track of
typed slots locally and then to merge them to the main remembered set
during finalization of marking.

The patch also cleans up atomics in the Iterate and ClearInvalidSlots
methods that can run concurrently to each other.

Bug:v8:8459

Change-Id: Id7a63041f7b99218381e5e9e1999210cab9c4369
Reviewed-on: https://chromium-review.googlesource.com/c/1340247
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57673}