Better codegen for the clone functions.

Previously, we first created the struct with zero's, then assigned
fields from the source one by one.

We now load all the fields from the source onto the stack, then
directly use `struct.new` to allocate the target.

It might not necessarily be more efficient, but it definitely
results is smaller code.

wasm/src/main/scala/org/scalajs/linker/backend/wasmemitter/ClassEmitter.scala

@@ -545,26 +545,28 @@ class ClassEmitter(coreSpec: CoreSpec) {
 
     val instrs = fb
 
-    val heapType = watpe.HeapType(genTypeID.forClass(className))
+    val structTypeID = genTypeID.forClass(className)
+    val structRefType = watpe.RefType(structTypeID)
 
-    val from = fb.addLocal("fromTyped", watpe.RefType.nullable(heapType))
-    val result = fb.addLocal("result", watpe.RefType.nullable(heapType))
+    val fromTypedLocal = fb.addLocal("fromTyped", structRefType)
 
+    // Downcast fromParam to fromTyped
     instrs += wa.LocalGet(fromParam)
-    instrs += wa.RefCast(watpe.RefType(heapType))
-    instrs += wa.LocalSet(from)
+    instrs += wa.RefCast(structRefType)
+    instrs += wa.LocalSet(fromTypedLocal)
 
-    instrs += wa.Call(genFunctionID.newDefault(className))
-    instrs += wa.LocalSet(result)
+    // Push vtable and itables on the stack (there is at least Cloneable in the itables)
+    instrs += wa.GlobalGet(genGlobalID.forVTable(className))
+    instrs += wa.GlobalGet(genGlobalID.forITable(className))
+
+    // Push every field of `fromTyped` on the stack
     info.allFieldDefs.foreach { field =>
-      val fieldIdx = genFieldID.forClassInstanceField(field.name.name)
-      instrs += wa.LocalGet(result)
-      instrs += wa.LocalGet(from)
-      instrs += wa.StructGet(genTypeID.forClass(className), fieldIdx)
-      instrs += wa.StructSet(genTypeID.forClass(className), fieldIdx)
+      instrs += wa.LocalGet(fromTypedLocal)
+      instrs += wa.StructGet(structTypeID, genFieldID.forClassInstanceField(field.name.name))
     }
-    instrs += wa.LocalGet(result)
-    instrs += wa.RefAsNotNull
+
+    // Create the result
+    instrs += wa.StructNew(structTypeID)
 
     fb.buildAndAddToModule()
   }