[gc_fuzz]: Add concrete struct types as fields (nullable)

[khagankhan]

References between struct types

• Concrete struct references — fields of type (ref null $t) pointing at any
  defined struct type, including self-references and cross-type cycles.
• Abstract structref fields — fields of type (ref null struct).
• Rec-group cycle merging — cross-group reference/supertype cycles are
  resolved by merging the strongly-connected rec groups into one (preserving all
  subtyping), instead of dropping supertype edges.

Scope / notes

• All references are nullable. Non-nullable references are deferred to a
  follow-up: a cycle of non-nullable references is uninhabitable, and detecting
  and repairing that in fixup is its own piece of work.
• Fixup degrades dangling concrete references to abstract structref and forces
  nullability, so every generated type stays constructible and valid.
• Bumped the default generation limits off zero so the fuzzer starts from a
  non-degenerate module.

+cc @fitzgen @eeide

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