This CL consists of several preparatory steps for slices in Torque. Above all, it introduces a user-defined struct, torque_internal::Slice<T>, that performs bounds checking and returns references to elements in arrays. To enable this, several smaller changes were also made:

- Constructors of internal classes such as torque_internal::Reference<T> now require a special 'Unsafe' argument, making it clear that there be dragons.
- Struct methods are now declared during finalization. This allows instances of generic structs to have methods referring to the same struct. Previously, methods would be declared before the instance had been fully registered, leading to errors during type resolution. Furthermore, such methods were declared in a temporary namespace, that would then erroneously escape and lead to use-after-free issues.
- Instances of TypeArgumentInference were not running in the correct (Torque) scopes, leading to type resolution errors.
- The chain of ContextualVariable::Scope for any given ContextualVariable (such as CurrentScope) can now be walked, simplifying debugging.

R=jgruber@chromium.org, tebbi@chromium.org

Bug: v8:7793
Change-Id: I36f808f63cc3ce441062dfc56f511f24f1e3121e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1758322
Commit-Queue: Georg Schmid <gsps@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63314}