gc_fuzz: Add eqref support

[khagankhan]

eqref:

types.rs:

• Added FieldType::EqRef to for_each_field_type! (storage (ref null eq), default ref.null eq) — picked up automatically by FieldType::ALL, to_storage_type, emit_default_const, is_packed, and random.
• Added StackType::Eq variant.
• Added an Eq arm to StackType::fixup that also accepts any Struct(_) on the stack (struct <: eq), emitting NullEq otherwise.

ops.rs:

• Extended WasmEncodingBases with eq_local_idx, eq_global_idx, eq_table_idx
• Added function take_eq the take_eq import, an eqref table, an eqref global, and an eqref local.
• Added take_eqref_idx = 4 and shifted typed-struct func-import base implicitly via imports.len().
• Added 8 new GcOp variants: NullEq, TakeEqCall, etc.

oracles.rs:

• Registered a take_eq host function taking (ref null eq) like take_struct.

+cc @fitzgen @eeide

[fitzgen]

Thanks!