wasm-ctor-eval: Handle cycles with non-nullable content

[kripken]

(module
 (type $A (sub final (struct (field (mut (ref struct))))))
 (type $B (struct ))

 (import "a" "b" (func $import))

 (func "test"
  (local $temp (ref $A))
  (struct.set $A 0
   (local.tee $temp
    (struct.new $A
     (struct.new_default $B)
    )
   )
   (local.get $temp)
  )
  ;; Stop evalling here, forcing us to serialize what we've seen so far in this
  ;; function, including its locals
  ;;
  ;; The local $temp contains a struct with a cycle now, and the cycle is on a
  ;; field that is mutable but *non*-nullable, so we can't use the usual trick
  ;; of writing a null initial value and fixing it up later. Instead, we need to
  ;; write some other type of value initially.
  (call $import)
 )
)

$ wasm-ctor-eval a.wat -all --ctors=test --kept-exports=test
trying to eval test
wasm-ctor-eval: wasm-ctor-eval.cpp:757: bool {anonymous}::CtorEvalExternalInterface::applyGlobalsToModule()::InitFixer::handleChild(wasm::Expression*&, wasm::Expression*, wasm::Index): Assertion `isNullableAndMutable(parent, fieldIndex)' failed.
Aborted

The input module cleverly creates a cycle using a non-nullable field by creating a temporary object just for that purpose. wasm-ctor-eval would need to be similarly clever, which seems difficult in general...