RTTs are internally represented as Maps. To store supertype information,
this patch introduces a WasmTypeInfo object, which is installed on Wasm
objects' Maps and points at both the off-heap type information and the
parent RTT.
In this patch, rtt.sub always creates a fresh RTT. The canonicalization
that the proposal requires will be implemented later.

Bug: v8:7748
Change-Id: I8286dd11f520966155cd95c2bd844ec34fccd131
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2260566
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68564}