Interop: add minor improvements

Interop/Runtime/src/main/kotlin/kotlin_native/interop/Types.kt

@@ -195,4 +195,8 @@ class Arena : Placement, Closeable {
 
     override fun close() = clear()
 
+}
+
+fun <T : NativeRef> Placement.alloc(type: NativeRef.TypeWithSize<T>): T {
+    return type.byPtr(this.alloc(type.size))
 }

Interop/Runtime/src/main/kotlin/kotlin_native/interop/Utils.kt

@@ -19,14 +19,16 @@ fun free(ptr: NativePtr?) {
 
 fun free(ref: NativeRef?) = free(ref.getNativePtr())
 
-fun <T : NativeRef> mallocNativeArrayOf(elemType: NativeRef.Type<T>, vararg elements: T?): NativeArray<RefBox<T>> {
-    val res = malloc(array[elements.size](elemType.ref))
+fun <T : NativeRef> Placement.allocNativeArrayOf(elemType: NativeRef.Type<T>, vararg elements: T?): NativeArray<RefBox<T>> {
+    val res = this.alloc(array[elements.size](elemType.ref))
     elements.forEachIndexed { i, element ->
         res[i].value = element
     }
     return res
 }
 
+fun <T : NativeRef> mallocNativeArrayOf(elemType: NativeRef.Type<T>, vararg elements: T?) = heap.allocNativeArrayOf(elemType, *elements)
+
 fun CString.Companion.fromString(str: String?): CString? {
     if (str == null) {
         return null