Struct locals etc

[khagankhan]

• Add support for generating locals, globals, and tables that contain structrefs and concretely typed struct references
• Add new or extend existing fuzzer ops for accessing locals/globals/tables of structrefs and concretely typed struct references

This PR implements the above changes. Open to feedback and reviews.

For now, all generated references are marked as nullable: true. As discussed, we will need to track nullability more precisely. I can either refine this within this PR after initial review, or follow up with a small, focused PR immediately afterward.

cc @fitzgen @eeide

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