[wasm] Use WeakFixedArray for list of instances sharing a WasmMemoryObject.

test/mjsunit/wasm/wasm-module-builder.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Used for encoding f32 and double constants to bits.
 let __buffer = new ArrayBuffer(8);
 let byte_view = new Int8Array(__buffer);
 let f32_view = new Float32Array(__buffer);
 let f64_view = new Float64Array(__buffer);
 
@@ skipping 605 matching lines @@
       view[i] = val | 0;
     }
     return buffer;
   }
 
   instantiate(ffi) {
     let module = new WebAssembly.Module(this.toBuffer());
     let instance = new WebAssembly.Instance(module, ffi);
     return instance;
   }
+
+  toModule(debug = false) {
+    return new WebAssembly.Module(this.toBuffer());
+  }
 }