smalloc: cleanup checks/conversions

* Moved the ToObject check out of smalloc::Alloc and into JS. Direct
  usage of that method is for internal use only and so can bypass the
  possible coercion.
* Same has been done with smalloc::SliceOnto.
* smalloc::CopyOnto will now throw if passed argument is not an object.
* Remove extra TargetFreeCallback function. There was a use for it when
  it was working with a Local<T>, but that code has been removed making
  the function superfluous.